Trench warfare

Breastwork "trench", Armentières, 1916

Australian light horseman using a periscope rifle, Gallipoli 1915

The banked earth on the lip of the trench facing the enemy was called the parapet and had a fire step. The embanked rear lip of the trench was called the parados, which protected the soldier's back from shells falling behind the trench. The sides of the trench were often revetted with sandbags, wire mesh, wooden frames and sometimes roofs. The floor of the trench was usually covered by wooden duckboards. In later designs the floor might be raised on a wooden frame to provide a drainage channel underneath.

The static movement of trench warfare and a need for protection from snipers created a requirement for loopholes both for discharging firearms and for observation.[26] Often a steel plate was used with a "key hole", which had a rotating piece to cover the loophole when not in use.[26] German snipers used armour-piercing bullets that allowed them to penetrate loopholes. Another means to see over the parapet was the trench periscope – in its simplest form, just a stick with two angled pieces of mirror at the top and bottom. A number of armies made use of the periscope rifle, which enabled soldiers to snipe at the enemy without exposing themselves over the parapet, although at the cost of reduced shooting accuracy. The device is most associated with Australian and New Zealand troops at Gallipoli, where the Turks held the high ground.

Dugouts of varying degrees of comfort were built in the rear of the support trench. British dugouts were usually 2.5 to 5 m (8 to 16 ft) deep. The Germans, who had based their knowledge on studies of the Russo-Japanese War,[27] made something of a science out of designing and constructing defensive works. They used reinforced concrete to construct deep, shell-proof, ventilated dugouts, as well as strategic strongpoints. German dugouts were typically much deeper, usually a minimum of 4 m (12 ft) deep and sometimes dug three stories down, with concrete staircases to reach the upper levels.

Trenches were never straight but were dug in a zigzagging or stepped pattern, with all straight sections generally kept less than a dozen meters (yards). Later, this evolved to have the combat trenches broken into distinct fire bays connected by traverses. While this isolated the view of friendly soldiers along their own trench, this ensured the entire trench could not be enfiladed if the enemy gained access at any one point; or if a bomb, grenade, or shell landed in the trench, the blast could not travel far.

Very early in the war, British defensive doctrine suggested a main trench system of three parallel lines, interconnected by communications trenches. The point at which a communications trench intersected the front trench was of critical importance, and it was usually heavily fortified. The front trench was lightly garrisoned and typically only occupied in force during "stand to" at dawn and dusk. Between 65 and 90 m (70 and 100 yd) behind the front trench was located the support (or "travel") trench, to which the garrison would retreat when the front trench was bombarded.

Between 90 and 270 metres (100 and 300 yd) further to the rear was located the third reserve trench, where the reserve troops could amass for a counter-attack if the front trenches were captured. This defensive layout was soon rendered obsolete as the power of artillery grew; however, in certain sectors of the front, the support trench was maintained as a decoy to attract the enemy bombardment away from the front and reserve lines. Fires were lit in the support line to make it appear inhabited and any damage done immediately repaired.

Aerial view of opposing trench lines between Loos and Hulluch, July 1917. German trenches at the right and bottom, British at the top-left.

Temporary trenches were also built. When a major attack was planned, assembly trenches would be dug near the front trench. These were used to provide a sheltered place for the waves of attacking troops who would follow the first waves leaving from the front trench. "Saps" were temporary, unmanned, often dead-end utility trenches dug out into no-man's land. They fulfilled a variety of purposes, such as connecting the front trench to a listening post close to the enemy wire or providing an advance "jumping-off" line for a surprise attack. When one side's front line bulged towards the opposition, a salient was formed. The concave trench line facing the salient was called a "re-entrant." Large salients were perilous for their occupants because they could be assailed from three sides.

Behind the front system of trenches there were usually at least two more partially prepared trench systems, kilometres to the rear, ready to be occupied in the event of a retreat. The Germans often prepared multiple redundant trench systems; in 1916 their Somme front featured two complete trench systems, one kilometre apart, with a third partially completed system a further kilometre behind. This duplication made a decisive breakthrough virtually impossible. In the event that a section of the first trench system was captured, a "switch" trench would be dug to connect the second trench system to the still-held section of the first.

French soldiers struggle to pass multiple lines of barbed wire

The use of lines of barbed wire, razor wire, and other wire obstacles, in belts 15 m (49 ft) deep or more, is effective in stalling infantry traveling across the battlefield. Although the barbs or razors might cause minor injuries, the purpose was to entangle the limbs of enemy soldiers, forcing them to stop and methodically pull or work the wire off, likely taking several seconds, or even longer. This is deadly when the wire is emplaced at points of maximum exposure to concentrated enemy firepower, in plain sight of enemy fire bays and machine guns. The combination of wire and firepower was the cause of most failed attacks in trench warfare and their very high casualties. Liddell Hart identified barbed wire and the machine gun as the elements that had to be broken to regain a mobile battlefield.

A basic wire line could be created by draping several strands of barbed wire between wooden posts driven into the ground. Loose lines of wire can be more effective in entangling than tight ones, and it was common to use the coils of barbed wire as delivered only partially stretched out, called concertina wire. Placing and repairing wire in no man's land relied on stealth, usually done at night by special wiring parties, who could also be tasked with secretly sabotaging enemy wires. The screw picket, invented by the Germans and later adopted by the Allies during the war, was quieter than driving stakes. Wire often stretched the entire length of a battlefield's trench line, in multiple lines, sometimes covering a depth 30 metres (100 ft) or more.

Methods to defeat it are rudimentary. Prolonged artillery bombardment could damage them, but not reliably. The first soldier meeting the wire could jump onto the top of it, hopefully depressing it enough for those that followed to get over him; this still took at least one soldier out of action for each line of wire. In World War I, British and Commonwealth forces relied on wire cutters, which proved unable to cope with the heavier gauge German wire.[28] The Bangalore torpedo was adopted by many armies, and continued in use past the end of World War II.[29]

The barbed wire used differed between nations; the German wire was heavier gauge, and British wire cutters, designed for the thinner native product, were unable to cut it.[28]

The confined, static, and subterranean nature of trench warfare resulted in it developing its own peculiar form of geography. In the forward zone, the conventional transport infrastructure of roads and rail were replaced by the network of trenches and trench railways. The critical advantage that could be gained by holding the high ground meant that minor hills and ridges gained enormous significance. Many slight hills and valleys were so subtle as to have been nameless until the front line encroached upon them. Some hills were named for their height in metres, such as Hill 60. A farmhouse, windmill, quarry, or copse of trees would become the focus of a determined struggle simply because it was the largest identifiable feature. However, it would not take the artillery long to obliterate it, so that thereafter it became just a name on a map.

The battlefield of Flanders presented numerous problems for the practice of trench warfare, especially for the Allied forces, mainly British and Canadians, who were often compelled to occupy the low ground. Heavy shelling quickly destroyed the network of ditches and water channels which had previously drained this low-lying area of Belgium. In most places, the water table was only a metre or so below the surface, meaning that any trench dug in the ground would quickly flood. Consequently, many "trenches" in Flanders were actually above ground and constructed from massive breastworks of sandbags filled with clay. Initially, both the parapet and parados of the trench were built in this way, but a later technique was to dispense with the parados for much of the trench line, thus exposing the rear of the trench to fire from the reserve line in case the front was breached.

In the Alps, trench warfare even stretched onto vertical slopes and deep into the mountains, to heights of 3,900 m (12,800 ft) above sea level. The Ortler had an artillery position on its summit near the front line. The trench-line management and trench profiles had to be adapted to the rough terrain, hard rock, and harsh weather conditions. Many trench systems were constructed within glaciers such as the Adamello-Presanella group or the famous city below the ice on the Marmolada in the Dolomites.

British Mills bomb N°23 Mk II, with rod for launch by rifle

At the start of the First World War, the standard infantry soldier's primary weapons were the rifle and bayonet; other weapons got less attention. Especially for the British, what hand grenades were issued tended to be few in numbers and less effective. This emphasis began to shift as soon as trench warfare began; militaries rushed improved grenades into mass production, including rifle grenades.

The hand grenade came to be one of the primary infantry weapons of trench warfare. Both sides were quick to raise specialist grenadier groups. The grenade enabled a soldier to engage the enemy without exposing himself to fire, and it did not require precise accuracy to kill or maim. Another benefit was that if a soldier could get close enough to the trenches, enemies hiding in trenches could be attacked. The Germans and Turks were well equipped with grenades from the start of the war, but the British, who had ceased using grenadiers in the 1870s and did not anticipate a siege war, entered the conflict with virtually none, so soldiers had to improvise bombs with whatever was available (see Jam Tin Grenade). By late 1915, the British Mills bomb had entered wide circulation, and by the end of the war 75 million had been used.

Since the troops were often not adequately equipped for trench warfare, improvised weapons were common in the first encounters, such as short wooden clubs and metal maces, spears, hatchets, hammers, entrenching tools, as well as trench knives and brass knuckles. According to the semi-biographical war novel All Quiet on the Western Front, many soldiers preferred to use a sharpened spade as an improvised melee weapon instead of the bayonet, as the bayonet tended to get "stuck" in stabbed opponents, rendering it useless in heated battle. The shorter length also made them easier to use in the confined quarters of the trenches. These tools could then be used to dig in after they had taken a trench. Modern military digging tools are as a rule designed to also function as a melee weapon. As the war progressed, better equipment was issued, and improvised arms were discarded.

Various trench weapons used by British and Canadian soldiers in WWI on display at the Canadian War Museum

French soldiers with a Sauterelle bomb-throwing crossbow, c. 1915

A specialised group of fighters called trench sweepers (Nettoyeurs de Tranchées or Zigouilleurs) evolved to fight within the trenches. They cleared surviving enemy personnel from recently overrun trenches and made clandestine raids into enemy trenches to gather intelligence. Volunteers for this dangerous work were often exempted from participation in frontal assaults over open ground and from routine work like filling sandbags, draining trenches, and repairing barbed wire in no-man's land. When allowed to choose their own weapons, many selected grenades, knives and pistols. FN M1900 pistols were highly regarded for this work, but never available in adequate quantities. Colt Model 1903 Pocket Hammerless, Savage Model 1907, Star Bonifacio Echeverria and Ruby pistols were widely used.[30]

Various mechanical devices were invented for throwing hand grenades into enemy trenches. The Germans used the Wurfmaschine, a spring-powered device for throwing a hand grenade about 200 m (220 yd).[31] The French responded with the Sauterelle and the British with the Leach Trench Catapult and West Spring Gun which had varying degrees of success and accuracy. By 1916, catapult weapons were largely replaced by rifle grenades and mortars.[32]

The Germans employed Flammenwerfer (flamethrowers) during the war for the first time against the French on 25 June 1915, then against the British 30 July in Hooge. The technology was in its infancy, and use was not very common until the end of 1917 when portability and reliability were improved. It was used in more than 300 documented battles. By 1918, it became a weapon of choice for Stoßtruppen (stormtroopers) with a team of six Pioniere (combat engineers) per squad.

Used by American soldiers in the Western front, the pump action shotgun was a formidable weapon in short range combat, enough so that Germany lodged a formal protest against their use on 14 September 1918, stating "every prisoner found to have in his possession such guns or ammunition belonging thereto forfeits his life", though this threat was apparently never carried out. The U.S. military began to issue models specially modified for combat, called "trench guns", with shorter barrels, higher capacity magazines, no choke, and often heat shields around the barrel, as well as lugs for the M1917 bayonet. Anzac and some British soldiers were also known to use sawn-off shotguns in trench raids, because of their portability, effectiveness at close range, and ease of use in the confines of a trench. This practice was not officially sanctioned, and the shotguns used were invariably modified sporting guns.

Vickers machine gun

The Germans embraced the machine gun from the outset—in 1904, sixteen units were equipped with the 'Maschinengewehr'—and the machine gun crews were the elite infantry units; these units were attached to Jaeger (light infantry) battalions. By 1914, British infantry units were armed with two Vickers machine guns per battalion; the Germans had six per battalion, and the Russians eight.[33] It would not be until 1917 that every infantry unit of the American forces carried at least one machine gun.[34] After 1915, the Maschinengewehr 08 was the standard issue German machine gun; its number "08/15" entered the German language as idiomatic for "dead plain". At Gallipoli and in Palestine the Turks provided the infantry, but it was usually Germans who manned the machine guns.

The British High Command were less enthusiastic about machine guns, supposedly considering the weapon too "unsporting" and encouraging defensive fighting; and they lagged behind the Germans in adopting it. Field Marshal Sir Douglas Haig is quoted as saying in 1915, "The machine gun is a much overrated weapon; two per battalion is more than sufficient".[35] The defensive firepower of the machine gun was exemplified during the first day of the Battle of the Somme when 60,000 British soldiers were rendered casualties, "the great majority lost under withering machine gun fire".[36] In 1915 the Machine Gun Corps was formed to train and provide sufficient heavy machine gun teams.

It was the Canadians that made the best practice, pioneering area denial and indirect fire (soon adopted by all Allied armies) under the guidance of former French Army Reserve officer Major General Raymond Brutinel. Minutes before the attack on Vimy Ridge the Canadians thickened the artillery barrage by aiming machine guns indirectly to deliver plunging fire on the Germans. They also significantly increased the number of machine guns per battalion. To match demand, production of the Vickers machine gun was contracted to firms in the United States. By 1917, every company in the British forces were also equipped with four Lewis light machine guns, which significantly enhanced their firepower.

The heavy machine gun was a specialist weapon, and in a static trench system was employed in a scientific manner, with carefully calculated fields of fire, so that at a moment's notice an accurate burst could be fired at the enemy's parapet or a break in the wire. Equally it could be used as light artillery in bombarding distant trenches. Heavy machine guns required teams of up to eight men to move them, maintain them, and keep them supplied with ammunition. This made them impractical for offensive manoeuvres, contributing to the stalemate on the Western Front.

German soldier with MP 18, 1918

One machine gun nest was theoretically able to mow down hundreds of enemies charging in the open through no man's land. However, while WWI machine guns were able to shoot hundreds of rounds per minute in theory, they were still prone to overheating and jamming, which often necessitated firing in short bursts.[37] However, their potential was increased significantly when emplaced behind multiple lines of barbed wire to slow any advancing enemy.

In 1917 and 1918, new types of weapons were fielded. They changed the face of warfare tactics and were later employed during World War II.

The French introduced the CSRG 1915 Chauchat during Spring 1916 around the concept of "walking fire", employed in 1918 when 250,000 weapons were fielded. More than 80,000 of the best shooters received the semi-automatic RSC 1917 rifle, allowing them to rapid fire at waves of attacking soldiers. Firing ports were installed in the newly arrived Renault FT tanks.

The French Army fielded a ground version of the Hotchkiss Canon de 37 mm used by the French Navy. It was primarily used to destroy German machine gun nests and concrete reinforced pillboxes with high explosive and armour-piercing rounds.

A new type of machine gun was introduced in 1916. Initially an aircraft weapon, the Bergmann LMG 15 was modified for ground use, with the later dedicated ground version being the LMG 15 n. A. It was used as an infantry weapon on all European and Middle Eastern fronts until the end of World War I. It later inspired the MG 30 and the MG 34 as well as the concept of the general-purpose machine gun.

What became known as the submachine gun had its genesis in World War I, developed around the concepts of infiltration and fire and movement, specifically to clear trenches of enemy soldiers when engagements were unlikely to occur beyond a range of a few feet. The MP 18 was the first practical submachine gun used in combat. It was fielded in 1918 by the German Army as the primary weapon of the stormtroopers – assault groups that specialised in trench combat. Around the same time, the Italians had developed the Beretta M1918 submachine gun, based on a design from earlier in the war.

Loading a 15 in (380 mm) howitzer

Artillery dominated the battlefields of trench warfare. An infantry attack was rarely successful if it advanced beyond the range of its supporting artillery. In addition to bombarding the enemy infantry in the trenches, the artillery could be used to precede infantry advances with a creeping barrage, or engage in counter-battery duels to try to destroy the enemy's guns. Artillery mainly fired fragmentation, high-explosive, shrapnel or, later in the war, gas shells. The British experimented with firing thermite incendiary shells, to set trees and ruins alight. However, all armies experienced shell shortages during the first year or two of World War I, due to underestimating their usage in intensive combat. This knowledge had been gained by the combatant nations in the Russo-Japanese War, when daily artillery fire consumed ten times more than daily factory output, but had not been applied.[38]

Artillery pieces were of two types: infantry support guns and howitzers. Guns fired high-velocity shells over a flat trajectory and were often used to deliver fragmentation and to cut barbed wire. Howitzers lofted the shell over a high trajectory so it plunged into the ground. The largest calibers were usually howitzers. The German 420 mm (17 in) howitzer weighed 20 tons and could fire a one-ton shell over 10 km (6.2 mi). A critical feature of period artillery pieces was the hydraulic recoil mechanism, which meant the gun did not need to be re-aimed after each shot, permitting a tremendous increase in rate of fire.

Initially each gun would need to register its aim on a known target, in view of an observer, in order to fire with precision during a battle. The process of gun registration would often alert the enemy an attack was being planned. Towards the end of 1917, artillery techniques were developed enabling fire to be delivered accurately without registration on the battlefield—the gun registration was done behind the lines then the pre-registered guns were brought up to the front for a surprise attack.

French soldiers operating a compressed-air trench mortar of 86-millimetre calibre

Mortars, which lobbed a shell in a high arc over a relatively short distance, were widely used in trench fighting for harassing the forward trenches, for cutting wire in preparation for a raid or attack, and for destroying dugouts, saps and other entrenchments. In 1914, the British fired a total of 545 mortar shells; in 1916, they fired over 6,500,000. Similarly, howitzers, which fire on a more direct arc than mortars, raised in number from over 1,000 shells in 1914, to over 4,500,000 in 1916. The smaller numerical difference in mortar rounds, as opposed to howitzer rounds, is presumed by many to be related to the expanded costs of manufacturing the larger and more resource intensive howitzer rounds.

The main British mortar was the Stokes, a precursor of the modern mortar. It was a light mortar, simple in operation, and capable of a rapid rate of fire by virtue of the propellant cartridge being attached to the base shell. To fire the Stokes mortar, the round was simply dropped into the tube, where the percussion cartridge was detonated when it struck the firing pin at the bottom of the barrel, thus being launched. The Germans used a range of mortars. The smallest were grenade-throwers ('Granatenwerfer') which fired the stick grenades which were commonly used. Their medium trench-mortars were called mine-throwers ('Minenwerfer'). The heavy mortar was called the 'Ladungswerfer', which threw "aerial torpedoes", containing a 200 lb (91 kg) charge to a range of 1,000 yd (910 m). The flight of the missile was so slow and leisurely that men on the receiving end could make some attempt to seek shelter.

Mortars had certain advantages over artillery such as being much more portable and the ability to fire without leaving the relative safety of trenches. Moreover, Mortar were able to fire directly into the trenches, which was hard to do with artillery.[39]

The main killer in the trenches was artillery fire; around 75 percent of known casualties.[46] Even if a soldier was not hit directly by the artillery, shell fragments and debris had a high chance of wounding those in close proximity to the blast. Artillery use increased tremendously during the war; for example, the percentage of the French army that was artillerymen grew from 20 percent in 1914 to 38 percent by 1918.[46] The second largest contributor to death was gunfire (bullets from rifles and machine-guns), which was responsible for 34 percent of French military casualties.[45]

Once the war entered the static phase of trench warfare, the number of lethal head wounds that troops were receiving from fragmentation increased dramatically. The French were the first to see a need for greater protection and began to introduce steel helmets in the summer of 1915. The Adrian helmet replaced the traditional French kepi and was later adopted by the Belgian, Italian and many other armies. At about the same time the British were developing their own helmets. The French design was rejected as not strong enough and too difficult to mass-produce. The design that was eventually approved by the British was the Brodie helmet. This had a wide brim to protect the wearer from falling objects, but offered less protection to the wearer's neck. When the Americans entered the war, this was the helmet they chose, though some units used the French Adrian helmet.

The predominant disease in the trenches of the Western Front was trench fever. Trench fever was a common disease spread through the faeces of body lice, who were rampant in trenches. Trench fever caused headaches, shin pain, splenomegaly, rashes and relapsing fevers - resulting in lethargy for months.[47] First reported on the Western Front in 1915 by a British medical officer, additional cases of trench fever became increasingly common mostly in the front line troops.[48] In 1921, microbiologist Sir David Bruce reported that over one million Allied soldiers were infected by trench fever throughout the war.[49] Even after the Great War had ended, disabled veterans in Britain attributed their decreasing quality of life to trench fever they had sustained during wartime.

Early in the war, gas gangrene commonly developed in major wounds, in part because the Clostridium bacteria responsible are ubiquitous in manure-fertilized soil[50] (common in western European agriculture, such as France and Belgium), and dirt would often get into a wound (or be rammed in by shrapnel, explosion, or bullet). In 1914, 12% of wounded British soldiers developed gas gangrene, and at least 100,000 German soldiers died directly from the infection.[51] After rapid advances in medical procedures and practices, the incidence of gas gangrene fell to 1% by 1918.[52]

Entrenched soldiers also carried many intestinal parasites, such as ascariasis, trichuriasis and tapeworm.[53] These parasites were common amongst soldiers, and spread amongst them, due to the unhygienic environment created by the common trench, where there were no true sewage management procedures. This ensured that parasites (and diseases) would spread onto rations and food sources that would then be eaten by other soldiers.[53]

Trench foot was a common environmental ailment affecting many soldiers, especially during the winter. It is one of several immersion foot syndromes. It was characterized by numbness and pain in the feet, but in bad cases could result in necrosis of the lower limbs. Trench foot was a large problem for the Allied forces, resulting in 2000 American and 75,000 British casualties.[54] Mandatory routine (daily or more often) foot inspections by fellow soldiers, along with systematic use of soap, foot powder, and changing socks, greatly reduced cases of trench foot.[55] In 1918, US infantry were issued with an improved and more waterproof 'Pershing boot' in an attempt to reduce casualties from trench foot.

To the surprise of medical professionals at the time, there was no outbreak of typhus in the trenches of the Western Front, despite the cold and harsh conditions being perfect for the reproduction of body lice that transmit the disease.[56] However, on the Eastern Front an epidemic of typhus claimed between 150,000 - 200,000 lives in Serbia.[57] Russia also suffered a globally unprecedented typhus epidemic during the last two years of the conflict that was exacerbated by harsh winters. This outbreak resulted in approximately 2.5 million recorded deaths, 100,000 of them being Red Army soldiers.[58] Symptoms of typhus include a characteristic spotted rash (which was not always present), severe headache, sustained high fever of 39 °C (102 °F), cough, severe muscle pain, chills, falling blood pressure, stupor, sensitivity to light, and delirium; 10% to 60% die. Typhus is spread by body lice.

Millions of rats called the trenches home, and were often responsible for the spread of diseases. Attempts to cull hordes of trench rats with rifle bayonets by soldiers were common early in the war, but the rats reproduced faster than they could be killed.[59] However, soldiers still partook in rat hunts as a form of entertainment. Rats would feed on half-eaten or uneaten rations, as well as corpses. Many soldiers were more afraid of rats than other horrors found in the trenches.[60]

Nervous and mental breakdowns amongst soldiers were common, due to unrelenting shellfire and the claustrophobic trench environment.[61] Men who suffered such intense breakdowns were often rendered completely immobile, and were often seen cowering low in the trenches, unable even to perform instinctive human responses such as running away or fighting back. This condition came to be known as "shell shock", "war neurosis" or "battle hypnosis".[62] Although trenches provided cover from shelling and small-arms fire, they also amplified the psychological effects of shell shock, as there was no way to escape a trench if shellfire was coming.[63] If a soldier became too debilitated from shell shock, they were evacuated from the trench and hospitalized if possible.[64] In some cases, shell shocked soldiers were executed for "cowardice" by their commanders as they became a liability.[65] This was often done by a firing squad composed of their fellow soldiers - often from the same unit.[66] Only years later would it be understood that such men were suffering from shell shock. 306 British soldiers were officially executed by their own side during this war.[67]

German Stoßtruppen (stormtroopers) rising from trenches to attack

As far back as the 18th century, Prussian military doctrine stressed maneuver and force concentration to achieve a decisive battle (Vernichtungsgedanke). The German military searched for ways to apply this in the face of trench warfare. Experiments with new tactics by Willy Rohr, a Prussian captain serving in the Vosges mountains in 1915, got the attention of the Minister of War. These tactics carried Prussian military doctrine down to smallest units — specially trained troops maneuvered and massed to assault positions they chose on their own.[68] During the next 2 years the German army tried to establish special stormtrooper detachments in all its units by sending selected men to Rohr and have those men then train their comrades in their original units.

Similar tactics were developed independently in other countries, such as French Army captain André Laffargue in 1915, and Russian general Aleksei Brusilov in 1916, but these failed to be adopted as any military doctrine.[69]

The German stormtrooper methods involved men rushing forward in small groups using whatever cover was available and laying down covering fire for other groups in the same unit as they moved forward. The new tactics, intended to achieve surprise by disrupting entrenched enemy positions, aimed to bypass strongpoints and to attack the weakest parts of an enemy's line. Additionally, they acknowledged the futility of managing a grand detailed plan of operations from afar, opting instead for junior officers on the spot to exercise initiative.[70]

The Germans employed and improved infiltration tactics in a series of smaller to larger battles, each increasingly successful, leading up to the Battle of Caporetto against the Italians in 1917, and finally the massive German Spring Offensive in 1918 against the British and French. German infiltration tactics are sometimes called "Hutier tactics" by others, after Oskar von Hutier, the general leading the German 18th Army, which had the farthest advance in that offensive. After a stunningly rapid advance, the offensive failed to achieve a breakthrough; German forces stalled after outrunning their supply, artillery, and reinforcements, which could not catch up over the shell-torn ground left ruined by Allied attacks in the Battle of the Somme in 1916. The exhausted German forces were soon pushed back in the Allied Hundred Days Offensive, and the Germans were unable to organise another major offensive before the war's end. In post-war years, other nations did not fully appreciate these German tactical innovations amidst the overall German defeat.

Explosion of a mine seen from a French position. 1916

Plan of the Y Sap mine

Mines - tunnels under enemy lines packed with explosives and detonated - were widely used in WWI to destroy or disrupt enemy's trench lines. Mining and counter-mining became a major part of trench warfare.[71][72]

The dry chalk of the Somme was especially suited to mining, but with the aid of pumps, it was also possible to mine in the sodden clay of Flanders. Specialist tunneling companies, usually made up of men who had been miners in civilian life, would dig tunnels under no man's land and beneath the enemy's trenches.[73][74] These mines would then be packed with explosives and detonated, producing a large crater. The crater served two purposes: it could destroy or breach the enemy's trench and, by virtue of the raised lip that they produced, could provide a ready-made "trench" closer to the enemy's line. When a mine was detonated, both sides would race to occupy and fortify the crater.

If the miners detected an enemy tunnel in progress, they would often counter-mine and try to drive a tunnel under the enemy's tunnel in which they would detonate explosives to create a camouflet to destroy the enemy's tunnel. Night raids were also conducted with the sole purpose of destroying the enemy's mine workings. On occasion, mines would cross and fighting would occur underground. The mining skills could also be used to move troops unseen. On one occasion a whole British division was moved through interconnected workings and sewers without German observation. The British detonated 19 mines of varying sizes on July 1, 1916, the first day of the Battle of the Somme. The largest mines—the Y Sap Mine and the Lochnagar Mine—each containing 24 tons of explosives, were blown near La Boiselle, throwing earth 4,000 feet into the air.

At 3.10 AM on June 7, 1917, a series of mines was detonated by the British to launch the Battle of Messines. The average mine contained 21 tons of explosive and the largest, 125 feet beneath Saint-Eloi, was twice the average at 42 tons. As remarked by General Plumer to his staff the evening before the attack:

"Gentlemen, we may not make history tomorrow, but we shall certainly change the geography."[75]

The craters from these and many other mines on the Western Front are still visible today. Two undetonated mines remained in the ground near Messines, with their location mislaid after the war. One blew during a thunderstorm in 1955; the other remains in the ground.[75] Significant mining operations were also carried out on the Italian Front.

Australian infantry wearing WWI gas masks, Ypres, September 1917

World War I saw large-scale use of poison gasses. At the start of the war, the gas agents used were relatively weak and delivery unreliable, but by mid-war advances in this chemical warfare reached horrifying levels.

The first methods of employing gas was by releasing it from a cylinder when the wind was favourable. This was prone to miscarry if the direction of the wind was misjudged. Also, the cylinders needed to be positioned in the front trenches where they were likely to be ruptured by enemy bombardment. Later, gas was delivered directly to enemy trenches by artillery or mortar shell, reducing friendly casualties significantly. Lingering agents could still affect friendly troops that advanced to enemy trenches following its use.

Early on, soldiers made improvised gas masks by urinating on a handkerchief and putting it over their nose and mouth so the urea would disable the poison. Armies rushed to issue regulation gas masks as regular equipment for front line troops. Anti-gas equipment and procedures improved significantly during the war, to the point that gas attacks had become less devastating at the war's end.

Several different gas agents were used. Tear gas was first employed in August 1914 by the French, but this could only temporarily disable the enemy. In April 1915, chlorine gas was first used by Germany at the Second Battle of Ypres. Exposure to a large dose could kill, and those not killed could suffer permanent lung damage. But the gas was easy to detect by scent and sight. Phosgene, first used in December 1915, was the most lethal killing gas of World War I; it was 18 times more powerful than chlorine and much more difficult to detect.

However, the most effective gas was mustard gas, introduced by Germany in July 1917. Mustard gas was not as fatal as phosgene, but it was hard to detect and lingered on the surface of the battlefield, so could inflict casualties over a long period. Even if not inhaled, it could slowly burn the skin, but quickly burned via the eyes or any wounds, causing blindness and intense suffering. Mustard gas also had the property of being heavier than air, causing it to sink down hills and therefore down into trenches. Casualties from mustard gas were unlikely to be fit to fight again, yet only 2% of mustard gas casualties died. The added burden of long-term care of casualties from mustard gas actually increased its overall effectiveness compared to more immediately lethal gas agents.

This British Mark IV tank displays a "tadpole tail" extension for crossing especially wide trenches, an experiment that was not successful

Failure of a tank to cross an anti-tank trench

Tanks were developed by the British and French as a means to attack enemy trenches, by combining heavy firepower (machine guns or light artillery guns), protection from small-arms fire (armour), and battlefield mobility (tracks). The British tanks were designed with a rhomboid shape, to easily surmount barbed wire and other obstacles. They were first deployed in 1916 at the Battle of the Somme in limited numbers, proving unreliable and ineffective at first, as mechanical and logistical issues overshadowed implementing a coherent tank doctrine, with the additional challenge of traversing ground torn apart by years of shell fire. At the First Battle of Cambrai in 1917, improved tanks in larger numbers demonstrated the potential of tank warfare, though German improvised anti-tank tactics, including using direct fire from field artillery, also proved effective.

By 1918, tank capabilities and tactics improved, their numbers increased and, combined with French tanks, finally helped break the stalemate. During the last 100 days of the war, Allied forces harried the Germans back using infantry supported by tanks and by close air support. By the war's end, tanks become a significant element of warfare; the proposed British Plan 1919 would have employed tanks as a primary factor in military strategy. However, the impact of tanks in World War I was less than it could have been, due to their late introduction and the inherent issues that plague implementing revolutionary technology.

Between the two world wars many nations developed their own unique tanks and divergent theories of tank tactics, including the UK, France, the Soviet Union, Czechoslovakia, Japan, the US, and Italy. Though German tank development was restricted by the terms of the treaty ending World War I, Germany successfully combined their own tanks (plus Czech tanks from occupied Czechoslovakia) with infiltration tactics to produce blitzkrieg during World War II.[76]

In the decade leading up to World War II, the French built the Maginot Line, based on their experience with trench warfare in World War I. The Maginot Line was an extensive state-of-the-art defensive system far superior to any previous trench system: a chain of massive constructions of concrete, iron, and steel fortresses, bunkers, retractable turrets, outposts, obstacles, and sunken artillery emplacements, linked by tunnel networks. It covered the length of the Franco-German border and was 20–25 kilometres (12–16 mi) wide. It was supported by numerous underground barracks, shelters, ammunition dumps and depots, with its own telephone network and narrow gauge railways with armoured locomotives, backed up with heavy rail artillery. French military experts placed high value on the line, saying it would curb German aggression, as any invasion force would be halted long enough for French forces to mobilize and counterattack. Furthermore, French military planning during the inter-war period believed that the line would force the Germans to invade Belgium. This would allow any future conflict to take place off of French soil. By bypassing the Maginot Line and fighting the Belgian Army, it would allow the French military to move its best formations to counter. In the Battle of France, German invaded Belgium and the best Anglo-French forces moved to meet them as planned. However, the Germans had only recently changed their plans from what the French had anticipated would happen. Instead of an attack through central Belgium, the main German attack was delivered through the Ardennes forest. Inter-war French planning believed it would take the Germans 9 days to move forces through this area, and that it could be held by small forces. The German forces outpaced expectations and soon crossed into France between the main French forces and the Maginot Line. They then advanced towards the English Channel, and surrounded the Anglo-French armies. Small secondary German attacks concentrated at a few points in the Line had mixed success. The bulk of the Maginot Line was untouched, its garrisons withdrawn, and flanked. Due to the lack of combat, much of it has survived.

The return of mobile warfare in World War II reduced the emphasis of trench warfare, as defenders commonly lacked the time to build up such battlefield defenses before they were forced to redeploy, due to the more rapidly-changing strategic situation. But trench systems were still effective, wherever mobility was limited, the front lines were static, or around known critical objectives that could not be bypassed. More quickly improvised defensive fighting positions, using "scrapes" or "foxholes", that can be supplemented by sand bags, local materials, debris, or rubble, remain in common use. These are typically improved and expanded by the defenders, eventually becoming full trench systems, if given enough time and resources.

A British trench mortar post in North Africa, 1940

Soviet soldiers running through the ruins of Stalingrad, 1942

At the Battle of Sevastopol, Red Army forces successfully held trench systems on the narrow peninsula for several months against intense German bombardment. The Western Allies in 1944 broke through the incomplete Atlantic Wall with relative ease through a combination of amphibious landings, naval gunfire, air attack, and airborne landings. Combined arms tactics where infantry, artillery, armour and aircraft cooperate closely greatly reduced the importance of trench warfare. It was, however, still a valuable method for reinforcing natural boundaries and creating a line of defence. For example, at the Battle of Stalingrad, soldiers on both sides dug trenches within the ruins, as well in the Battle of Hurtgen Forest, both American and German soldiers also dug trenches in the rugged woods of the forest which lead to continuous stalemates and failed offensives that lasted for months, which was reminiscent of the trench warfare of World War I. In addition, before the start of the Battle of Kursk, the Soviets constructed a system of defence more elaborate than any they built during World War I. These defences succeeded in stopping the German armoured pincers from meeting and enveloping the salient.[77]

The Italian Campaign fought from 1943 until the end of the war in Europe largely consisted of the Allies storming strongly fortified German lines which stretched from one coast, over the mountains to the other coast. When the Allies broke through one line, the Germans would retreat up the peninsula to yet another freshly prepared fortified line.

At the start of the Battle of Berlin, the last major assault on Germany, the Russians attacked over the river Oder against German troops dug in on the Seelow Heights, about 50 km (31 mi) east of Berlin. Entrenchment allowed the Germans, who were massively outnumbered, to survive a bombardment from the largest concentration of artillery in history; as the Red Army attempted to cross the marshy riverside terrain they lost tens of thousands of casualties to the entrenched Germans before breaking through.

During the Pacific War, the Japanese used a labyrinth of underground fixed positions to slow down the Allied advances on many Pacific Islands. The Japanese built fixed fortifications on Iwo Jima, Okinawa, and Peleliu using a system of tunnels to interconnect their fortified positions. Many of these were former mine shafts that were turned into defense positions. Engineers added sliding armored steel doors with multiple openings to serve both artillery and machine guns. Cave entrances were built slanted as a defense against grenade and flamethrower attacks. The caves and bunkers were connected to a vast system throughout the defenses, which allowed the Japanese to evacuate or reoccupy positions as needed, and to take advantage of shrinking interior lines. This network of bunkers, tunnels, and pillboxes favored the defense. For instance, the Japanese had on Battle of Iwo Jima several levels of honeycombed fortifications. The Nanpo Bunker (Southern Area Islands Naval Air HQ), which was located east of Airfield Number 2, had enough food, water and ammo for the Japanese to hold out for three months. The bunker was 90 feet deep and had tunnels running in various directions. Approximately 500 55-gallon drums filled with water, kerosene, and fuel oil for generators were located inside the complex. Gasoline powered generators allowed for radios and lighting to be operated underground.[78] The Japanese caused the American advance to slow down and caused massive casualties with these underground fixed positions. The Americans eventually used flamethrowers and systematic hand-to-hand fighting to oust the defenders.[79][80] The American ground forces were supported by extensive naval artillery, and had complete air supremacy provided by U.S. Navy and Marine Corps aviators throughout the entire battle.[81]

Iranian Troops in forward trenches during the Iran–Iraq War

Trench warfare has been infrequent in recent wars. When two large armoured armies meet, the result has generally been mobile warfare of the type which developed in World War II. However, trench warfare re-emerged in the latter stages of the Chinese Civil War (Huaihai Campaign) and the Korean War (from July 1951 to its end).

During the Cold War, NATO forces routinely trained to fight through extensive works called "Soviet-style trench systems", named after the Warsaw Pact's complex systems of field fortifications, an extension of Soviet field entrenching practices for which they were famous in their Great Patriotic War (the Eastern Front of World War II).

Ukrainian soldier in the trenches during the War in Donbass

An example of trench warfare after World Wars I and II was the Iran–Iraq War, in which both armies lacked training in combined arms operations. However, both countries often prepared entrenched defensive positions and tunnels to protect and supply the cities and bases throughout the regions. Another problem and success from both sides was their military mobility had reduced, drastically, due to hidden various land mines, and with no firm or stable ground footing it was easy to slide down or even get buried in a camouflaged anti-tank trench. Tactics used included trench warfare, machine gun posts, bayonet charges, booby traps, use of barbed wire across trenches and on no-man's land, Iranian human wave attacks, and Iraq's extensive use of chemical weapons such as mustard gas against Iranian troops.[82]

Afghan and U.S. soldiers provide security while standing behind a blast wall made from HESCO bastions at Zhari district, Kandahar Province, Afghanistan, 2012

Although mainly a siege, it was not unusual to find an extensive trench system inside and outside the city of Sarajevo during the siege of 1992–1996. It was used mainly for transportation to the front-line or to avoid snipers inside the city. Any pre-existing structures were used as trenches; the best known example is the bobsleigh course at Trebević, which was used by both Serb and Bosniak forces during the siege.

Another example of trench stalemate was the Eritrean-Ethiopian War of 1998–2000. The widespread use of trenches has resulted in comparisons of the conflict to the trench warfare of World War I.[83] According to some reports, trench warfare led to the loss of "thousands of young lives in human-wave assaults on Eritrea's positions".[84] The Eritrean defences were eventually overtaken by a surprise Ethiopian pincer movement on the Western front, attacking a mined, but lightly defended mountain (without trenches), resulting in the capture of Barentu and an Eritrean retreat. The element of surprise in the attack involved the use of donkeys as pack animals as well as being a solely infantry affair, with tanks coming in afterwards only to secure the area.[85]

The front line in Korea and the front lines between Pakistan and India in Kashmir are two examples of demarcation lines which could become hot at any time. They consist of kilometres of trenches linking fortified strongpoints and in Korea surrounded by millions of land mines. The borders between Armenia, Republic of Artsakh and Azerbaijan amid the ongoing Nagorno-Karabakh conflict are also heavily fortified with trenches and barbed wire, with the two sides regularly trading fire.[86]

A recent example of trench warfare is the War in Donbass. To consolidate and defend their territory, government and rebel forces have dug trenches and have initiated warfare reminiscent of World War I, with troops staying in and fighting from trenches for months at a time using cement mixers and excavators at their lines for digging network tunnels and deep bunkers for protection.[87] Since the Minsk peace agreements the front lines have not moved significantly and the two sides mostly fire mortars and sniper shots at each other. As a result, both sides have dug elaborate networks of trenches and deep bunkers for protection.[88]