[1] 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 first U-boat sent—U-21—achieved initial success, sinking the Royal Navy predreadnought battleships HMS Triumph and Majestic 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.

When operating in the west, up to the line of Cape Matapan, the German U-boats flew the Austrian flag, and a sinking without warning policy was adopted, since large merchant ships could be attacked on the suspicion of being transports or auxiliary cruisers.

This meant a shortage of workers to complete U-boats for service in home waters, but it seemed justified by the successes in the Mediterranean in November, when 44 ships were sunk, for a total of 155,882 long tons (158,383 t).

In November 1915, U-38—sailing under the Austrian flag and commanded by Kapitanleutnant (K/L) Max Valentiner—caused a diplomatic incident when she sank the Italian passenger liner SS Ancona off the coast of Tunisia.

Coming as it did six months after the sinking of the British liner RMS Lusitania off Ireland, the Ancona incident added to a growing outrage in the U.S. over unrestricted submarine warfare, and U.S. Secretary of State Robert Lansing despatched a sternly-worded protest to Vienna.

This too was ineffective; the straits were too wide and deep for such a barrage to be successful, and consumed a huge effort and tied up many of the patrol vessels the Allies possessed.

Eight of the top twelve U-boat aces served in the Pola Flotilla, including the highest scoring commander of all, K/L Arnauld de la Perière.

In January 1917, following the German decision to resume unrestricted submarine warfare, Foreign Secretary Arthur Zimmermann led a delegation to Vienna to secure the collaboration of Austria-Hungary.

Grand Admiral Haus wholly supported the proposal, but Foreign Minister Count Ottokar Czernin had misgivings, as did the emperor, Charles I of Austria.

Haus and the German delegates finally won the debate, partly by listing several instances where Allied submarines had sunk unarmed Austro-Hungarian ships in the Adriatic.

The negotiations over the terms of the new Mediterranean submarine campaign were aided by the fact that Italy had declared war on Germany on 28 August 1916, making it no longer necessary for German U-boats to masquerade as Austrian vessels when attacking Italian shipping.

Beginning in April 1917, Japan, an ally of Great Britain, sent a total of 14 destroyers to the Mediterranean with cruiser flagships which were based at Malta and played an important part in escorting convoys to guard them against enemy submarines.

The number of routes, and divided responsibilities, made this complicated, while a continued belief in offensive measures, such as the Otranto Barrage, kept up a shortage of escort ships elsewhere.