The Mediterranean U-boat Campaign in the Mediterranean Sea was fought by Austria-Hungary and German Empire (with some support by the Ottoman Empire) against the Allies during World War I. It was characterised by the ability of the Central Powers to raid with near impunity during the first years of the war, causing substantial shipping losses, until the introduction of the convoy system allowed the Allies to drastically cut their losses from 1917 on.
History
1914: Initial stages
At the outbreak of World War I, with Italy's decision to remain neutral, the naval strength of the Central Powers was represented by the navy of the Austro-Hungarian Empire, the KuK Kriegsmarine, whose only access to the sea was through the Adriatic coast. The Entente powers moved swiftly to blockade the Adriatic, sending a fleet to take station at the straits of Otranto.
The initial phase of the U-boat campaign in the Mediterranean comprised the actions by the KuK's U-boat force against the French. At the start of hostilities, the KuK had seven U-boats in commission; five operational, and two training; all were of the coastal type, with limited range and endurance, suitable for operation in the Adriatic.
Nevertheless, they had a number of successes. On 21 December 1914, torpedoed the French battleship , Admiral Lapeyrere's flagship. She was saved from sinking, but was forced to retire with a damaged bow. This setback dissuaded the French capital ships from penetrating too far into the Adriatic. On 27 April 1915, sank the French cruiser , with heavy loss of life.
But the KuK boats were unable to offer any interference to allied traffic in the Mediterranean beyond the Straits of Otranto.
1915
Operations at the Dardanelles
In April 1915, the Imperial German Navy sent their first submarines to the Mediterranean in response to the Anglo-French Dardanelles campaign, after it became obvious that their Austro-Hungarian allies could do little against it with their small submarine force, which nevertheless was successful in defending the Adriatic.
The first U-boat sent——achieved initial success, sinking the Royal Navy predreadnought battleships and on 25 and 27 May respectively on her way to Constantinople, but ran into severe limitations in the Dardanelles, where swarms of small craft and extensive anti-submarine netting and booms restricted their movements. In addition, the Germans dispatched a number of UB and UC type boats; these were sent in sections by rail to Pola where they were assembled for transit to Constantinople. One was lost, but by the end of 1915 the Germans had established a force of seven U-boats at Constantinople, misleadingly named the Mediterranean U-Boat Division.
Pola Flotilla
At the same time, the Germans determined to establish a force in the Adriatic to open the commerce war against Allied trade in the Mediterranean.
By the end of June 1915, the Germans had assembled a further three pre-fabricated Type UB I submarines at Pola in Istria, two of them intended for transfer to the Austrian Navy. They were also assembling three Type UC I minelaying submarines, which were ordered converted into transports to carry small quantities of critical supplies to Turkey. However, the UB submarines were hindered by their short operational range and the Dardanelles currents, and in July U-21—the only U-boat with a decent operating range—was damaged by a mine and confined to Constantinople.
On 21 July, the ocean-going submarines and were detached from service in the Baltic and sent to Cattaro (in present-day Montenegro), the Germans deciding to make use of Austrian bases rather than Constantinople, since there were better supply and repair facilities in the Adriatic and it avoided submarines having to negotiate the dangerous passage through the Dardanelles. In August, and joined the German flotilla stationed at Cattaro, following pleas from the German military attaché in Constantinople, who reported that the Royal Navy's close naval support was inflicting heavy losses on Turkish forces at the Gallipoli beachheads.