Yugoslav monitor Drava

Soon after Enns was commissioned into the Danube Flotilla she was in action against Serbian forces at Belgrade,[8] under the command of Linienschiffsleutnant[lower-alpha 3] (LSL) Richard Funk.[10] French artillery support arrived in Belgrade in November, endangering the monitor's anchorage, with Enns being the only monitor in the flotilla with the range to match the French guns.[8] On 21 November, Enns engaged the French battery in an artillery duel at a range of 10 km (6.2 mi).[10] This stalemate continued until the following month when the Serbs briefly evacuated Belgrade in the face of an Austro-Hungarian assault. After less than two weeks, the Austrians had to withdraw from Belgrade, and it was soon re-occupied by the Serbs, reinforced by the Russians and French. Enns continued in action against Serbia and her allies at Belgrade until late December, when the monitor base was withdrawn to Petrovaradin for the winter.[11]

In January 1915, British artillery arrived in Belgrade, further bolstering its defences.[12] In mid-February, the monitors redeployed to an anchorage at Zemun.[10] Following the commencement of the Gallipoli campaign, munitions supply to the Ottomans became critical, and despite the failure of an earlier attempt to ship arms and ammunition down the Danube, another attempt was planned. On 30 March, the steamer Belgrad left Zemun, escorted by Enns and the monitor SMS Bodrog. The convoy was undetected as it sailed past Belgrade at night during a storm, but after the monitors returned to base, the steamer struck a mine near Vinča, and after coming under heavy artillery fire, exploded near Ritopek.[13] On 22 April 1915, a British picket boat that had been brought overland by rail from Salonika was used to attack the Danube Flotilla anchorage at Zemun, firing two torpedoes without success.[14] In September 1915, the Central Powers were joined by Bulgaria, and the Serbian Army soon faced overwhelming Austrian and German ground troops. In early October, the Austro-Hungarian Third Army attacked Belgrade, and Enns, along with the majority of the flotilla, was heavily engaged in support of the crossings near the Belgrade Fortress and Ada Ciganlija island.[15] During the final river crossing and support of the resulting bridgehead, Enns was near Grosser Krieg Island on 8 October when she received a direct hit below the waterline and her 120 mm (4.7 in) magazine flooded. She was towed out of danger by the armed steamer Almos, and was eventually hauled to Budapest where she was repaired.[16] During repairs, the barbettes on her upper deck were replaced with turrets for the anti-aircraft guns. Although the howitzers had not been particularly successful, a plan to replace the aft howitzer mount with a turret similar to that used on SMS Bodrog was not carried out.[1]

View from the Belgrade Fortress over Grosser Krieg Island. Enns was supporting the October 1915 crossings of the Danube near the island when she received a direct hit and was put out of action

When she returned to the flotilla after repairs, she saw action at Rjahovo in early October 1916, where she contributed to the defeat of the Romanian Flămânda Offensive. The Romanian attempt to cross the Danube to attack the rear of Generalfeldmarschall[lower-alpha 4] August von Mackensen's Austro-Hungarian Third Army was thwarted,[18] and a force consisting of Enns, the monitors SMS Leitha, SMS Temes (II) and SMS Szamos, the patrol boat Viza and the armed steamer Balaton destroyed a pontoon bridge near Rjahovo. After forays against Giurgiu to secure trains loaded with coal and oil, in November Enns and other ships supported the crossing of the Danube by von Mackensen's army at Sistow. The following month, Enns bombarded Căscioarele, driving enemy troops out of the village. From late December 1916 to mid-March 1917, Enns and other ships of the flotilla wintered at Turnu Severin.[10]

In March 1917, Enns relocated to Brăila in eastern Romania, where it remained until July 1918. Sent to Linz and Budapest for an overhaul in dry dock, Enns then returned to eastern Romania and was stationed at Reni where she met a group of monitors and patrol boats that had been operating against Russia in the Black Sea.[10] In October 1918, the Danube Flotilla was under serious threat of being cut off in the lower Danube by French forces after the Bulgarians concluded an armistice with the British and French. After the steamer Croatia was fired on by the French as it tried to get past Lom, she cut her tow line, releasing seven lighters, which ran aground on a sandbank. Croatia was hit, suffered casualties and grounded on the Romanian side of the river. The French retrieved three of the lighters and towed them into the anchorage at Lom. The following day, Enns and two other monitors managed to release three of the remaining lighters while under heavy French fire, and towed them upstream.[19] The flotilla continued to retreat up the Danube, running the gauntlet of French and Serbian forces. With the dissolution of Austria-Hungary in early November, the South Slavs went ashore at Vukovar. The Austrian, Hungarian and Czech crew members of the flotilla continued on their journey, arriving in Budapest on 6 November,[20] and Enns began flying the Hungarian flag,[10] as part of the navy of the Hungarian People's Republic.[21] On 8 December, the monitors were seized by the Allies, and less than two weeks later Enns was towed to Belgrade where she was handed over to Serbia to be maintained on behalf of the new Kingdom of Serbs, Croats and Slovenes (KSCS, later the Kingdom of Yugoslavia).[10]

Immediately after the armistice, Enns was crewed by sailors of the KSCS in 1918–19. Under the terms of the Treaty of Saint-Germain-en-Laye concluded in September 1919, Enns was transferred to the KSCS along with a range of other vessels, including three other river monitors,[22] and was officially handed over to the KSCS navy and renamed Drava in 1920.[23] Her sister ship Inn was transferred to Romania and renamed Besarabia.[3] In 1925–26, Drava was refitted, but by the following year only two of the four river monitors of the KSCS Navy were being retained in full commission at any time.[24] In 1932, the British naval attaché reported that Yugoslav ships were engaging in little gunnery training, and few exercises or manoeuvres, due to reduced budgets.[25]

Ju 87 Stuka dive bombers of Sturzkampfgeschwader 77 sank Drava near Čib on the morning of 12 April 1941

Drava was based at Bezdan under the command of Aleksandar Berić,[lower-alpha 5] when the German-led Axis invasion of Yugoslavia began on 6 April 1941. She was assigned as flagship of the 1st Mine Barrage Division,[27] and was responsible for the Hungarian border on the Danube, under the operational control of the 30th Infantry Division Osiječka,[28] which was part of the 2nd Army.[29] Drava steamed upstream to Mohács in Hungary to shell the airfield there on 6 and 8 April,[30] but was subjected to daily attacks by the Luftwaffe.[31] On 10 April, Drava and her fellow monitor Morava were ordered to sail downstream to conform with the withdrawals of the 1st and 2nd Army's from Bačka and Baranja.[29] About 14:00 the following day, a Yugoslav lookout near Batina signalled Drava that a group of four Hungarian patrol boats, armed with 70 mm (2.8 in) guns, was coming down the Danube from the direction of Mohács. Drava engaged the patrol boats at a range of 6–7 km (3.7–4.3 mi), and drove the small Hungarian flotilla north again. Berić followed this up at 16:00 by once again shelling the airfield at Mohács. Morale on the ship was good, but when Berić met with Army elements later that day he became aware of the situation elsewhere, and nine crew deserted.[32]

Early on 12 April, with the other three monitors having been scuttled the night before, Drava was attacked by Junkers Ju 87 Stuka dive bombers of Sturzkampfgeschwader 77 flying from Arad, Romania. The anti-aircraft gunners on the ship claimed three enemy aircraft, and nine of the Stukas scored hits on her, most of which had little effect. However, the last bomb dropped straight down her funnel and exploded in her engine room, killing 54 of the crew. Only 13 survived,[30] and she sank off Čib.[31] Having ordered the burning of codes before she sank, Berić and his first officers were among the dead, but two of the successful anti-aircraft gunners, Rade Milojević and Miroslav Šurdilović, survived.[33] During their occupation of parts of Yugoslavia, Drava was raised and then scrapped by Hungary.[34] Berić was posthumously awarded the Order of Karađorđe's Star for his sacrifice. In April 2015, a bust of Berić was unveiled at the village of Belegiš, near Stara Pazova.[35] The barracks of the Serbian River Flotilla in Novi Sad are also named after him.[33]