In early 1915, U-21 was transferred to the Mediterranean Sea to support the Ottoman Empire against the Anglo-French attacks during the Gallipoli Campaign.

Throughout 1916, U-21 served in the Austro-Hungarian Navy as U-36, since Germany was not yet at war with Italy and thus could not legally attack Italian warships under the German flag.

The boat's propulsion system consisted of a pair of 8-cylinder 2-stroke diesel engines manufactured by MAN for use on the surface, and two electric double motor-dynamos built by AEG for use while submerged.

[6] At the outbreak of World War I in August 1914, U-21 was based at Heligoland in the German Bight, commanded by Kapitänleutnant (Captain Lieutenant) Otto Hersing.

The patrol was an attempt to locate the British blockade line and gather intelligence, but they spotted only a single cruiser and a destroyer off the Norwegian coast.

Hersing had surfaced his U-boat to recharge his batteries when a lookout spotted smoke from Pathfinder's funnels on the horizon.

Hersing manoeuvred into an attack position and fired a single torpedo, which hit Pathfinder just aft of her conning tower.

U-21's next success came three days later with the British collier SS Primo, which he also sank in accordance with the cruiser rules that governed commerce raiding.

The next week, U-21 stopped the collier SS Ben Cruachan; after evacuating her crew, the Germans sank her with scuttling charges.

In both cases, Hersing adhered to the prize rules, including flagging down a passing trawler to pick up the ships' crews.

[16] Hersing took his submarine north around Scotland to avoid the Dover patrols, and rendezvoused with the supply ship SS Marzala off Cape Finisterre to refuel.

Unfortunately for the Germans, Marzala carried poor quality oil that could not be burned in the boat's diesel engines; U-21 had less than half of her fuel supply remaining, and was only halfway on the voyage to Austria-Hungary.

While en route the Germans managed to avoid patrolling British and French torpedo boats and transport ships that might have reported their location.

[16] Several other German submarines joined U-21 in the following months, after calls for assistance from the Ottoman ground forces on the Gallipoli peninsula, who were taking heavy casualties from the bombardments from Allied warships.

[20] These two successes brought significant dividends: all Allied capital ships were withdrawn to protected anchorages and were thus unable to bombard Ottoman positions on the peninsula.

[22] After sinking Majestic, Hersing took his submarine to refuel at a Turkish port before attempting the dangerous route through the Dardanelles to Constantinople.

Later on the patrol, a lookout on an Allied trawler spotted U-21's periscope; the Germans had to crash-dive to escape being rammed, but doing so brought them into a minefield.

In the meantime, the Allies had established a complete blockade of the Dardanelles with mines and nets to prevent submarines from operating out of Constantinople.

As a result, German U-boats could not legally attack Italian ships, despite the fact that Italy was at war with Austria-Hungary.

[27] In early 1916, while patrolling off Sicily, U-36 encountered an Allied Q-ship, an auxiliary cruiser disguised as an unarmed merchant ship.

On 23 December, U-21 torpedoed the British steamer SS Benalder east of Crete, but the ship managed to reach Alexandria.

[1][29] Hersing attacked a convoy of fifteen merchant ships escorted by fourteen destroyers in August south-west of Ireland.

[30] The experience led Hersing to change tactics in future attacks on escorted convoys; instead of attacking the ships from as far away as possible, he chose to fire his torpedoes at closer range and then dive under the transport ships, where the destroyers would be unable to launch their depth charges for fear of damaging the transports.

She survived the war, but on 22 February 1919, she accidentally sank in the North Sea while under tow to Britain, where she was to be formally surrendered.