Henry Horne, 1st Baron Horne

In November 1915, Horne accompanied Lord Kitchener to the Dardanelles, where they organised and executed the evacuation of Gallipoli. For several months, Horne was placed in charge of the Suez Canal defences (and given command of the XV Corps).[3]

A map of the first day of the Battle of the Somme. Horne's forces were located on the right flank of the attacking line.

March 1916 saw him return to the Western Front. He was allotted to the Fourth Army, which was preparing for an attack in the Somme area. In the pre-battle plans, Horne advocated and became an architect of the "creeping barrage", a tactic that was used for the rest of the war. On 1 July 1916, Horne's XV Corps participated in the costliest battle of the First World War. His force consisted of the 7th and 21st Divisions. They attacked the villages of Fricourt and Mametz, capturing both on the first day although suffering 7,500 casualties in the process.

The divisions bypassed Mametz Wood, a position the Germans had heavily entrenched and needed to be captured to allow XV to carry on the advance. As the 7th Division had suffered heavy casualties, the 38th (Welsh) Division was assigned to the Corps and ordered to take the wood. Horne interfered in the division's efforts to attack the wood, issuing conflicting orders and going as far as to ordering a single platoon into action. Due to the miscommunication between Horne and the division's commander, Ivor Philipps was fired and replaced by the commander of the 7th Division. Horne wrote a "self-serving" account of this event that did no justice to men of the division or the difficulties they had faced. On 9 July the Welsh launched a full-scale attack on the woods and cleared it by the following day. During their 6 days on the Somme, the Welsh division suffered 3,993 casualties. Historian Don Farr wrote that the reputation of the Welsh division suffered due to the repeated interference by Horne in matters best left to the divisional or brigade staff and his "inexperience of battlefield command at this level". With the wood cleared, Horne would lead his Corps during the Bazentin Ridge, the Battle of Delville Wood, and the Battle of Flers–Courcelette.

In September 1916, Horne was created a Knight Commander of the Order of the Bath. After the successful capture of Flers, he was promoted to temporary General and succeeded Sir Charles Monro (after a brief hiatus between 7 August and 29 September when the command was held by Haig's first choice Richard Haking, who was then blocked from receiving the promotion) as commander of the First Army.[3] On 1 January 1917, he was promoted to the substantive rank of Lieutenant-General "for distinguished service in the field".[6] His first trial occurred in April 1917, when his troops were sent on a diversionary attack on the fearsome Vimy Ridge, which rose hundreds of feet over the surrounding landscape. French Army commander Robert Nivelle was critical of Horne's plan; Nivelle was the one found incompetent and, after one month of relative failure (and a mutiny), Nivelle was sacked and replaced with Philippe Pétain.

The attack on Vimy Ridge was spearheaded by the First Army's "shock troops" (the Canadian Corps). The ensuing Battle of Vimy Ridge, the first of a series of actions known as the Battle of Arras was successful: supported by Horne's 1,000-odd artillery pieces, the Canadian forces took the ridge in four days, with approximately 10,000 casualties (against 20,000 German casualties). The capture of Vimy Ridge would prove essential to the British Army: it served as the backbone of the British defence from March 1918 onward.

Nivelle's failure and sacking lengthened the actions around Arras. With success imminent, Haig began siphoning troops northward, where many would participate in the Battles of Messines and Passchendaele. The First Army served mainly as a diversion and a placeholder until April 1918.

Henry Horne, the Commander of the First Army, inspecting the 24th Motor Machine Gun Battalion at Dieval, 12 June 1918. The motorbikes are Clyno 744 cc twin cylinder machines fitted with a sidecar and Vickers machine-guns.

In April, the Germans embarked on the Spring Offensive which was similar to the Allied Somme Offensive two years previously. At first, the attack was successful. On Horne's front, nine German divisions attacked his weak left flank which was manned by two exhausted Portuguese divisions. The Germans advanced six miles to the banks of the River Lawe, where they were repulsed by the 55th (West Lancashire) and the 51st (Highland) Divisions.

After this final German offensive, the British took the initiative permanently. Haig's forces embarked on the Hundred Days Offensive, which ended the war; Horne's troops distinguished themselves in the lengthy offensive.