nets, and 30 motor launches, all equipped with depth charges, was designed to stop the passage of U-boats from Cattaro. However, this failed to do so, and from its inception in 1916, the barrage had caught only two U-boats, the Austrian and the German out of hundreds of possible passages. However, the barrage effectively meant that the Austro-Hungarian surface fleet could not leave the Adriatic Sea unless it was willing to give battle to the blocking forces. This, and as the war drew on bringing supply difficulties especially coal, plus a fear of mines, limited the Austro-Hungarian navy to shelling the Italian and Serbian coastlines. There had already been four small-scale Austro-Hungarian attacks on the barrage, on 11 March, 21 and 25 April and 5 May 1917, but none of them amounted to anything. Now greater preparations were made, with two U-boats despatched to lay mines off Brindisi with a third patrolling the exits in case Anglo-Italian forces were drawn out during the attack. The whole operation was timed for the night of 14/15 May, which led to the biggest battle of the Austro-Hungarian navy in World War I, the Battle of the Otranto Straits. The first Austro-Hungarian warships to strike were the two destroyers, and . An Italian convoy of three ships, escorted by the destroyer , was approaching Valona, when, out of the darkness, the Austrians fell upon them. Borea was left sinking. Of the three merchant ships, one loaded with ammunition was hit and blown up, a second set on fire, and the third hit. The two Austrian destroyers then steamed off northward. Meanwhile, three Austro-Hungarian cruisers under the overall command of Captain Miklós Horthy, , , and , had actually passed a patrol of four French destroyers north of the barrage, and thought to be friendly ships passed unchallenged. They then sailed through the barrage before turning back to attack it. Each Austrian cruiser took one-third of the line and began slowly and systematically to destroy the barrage with their guns, urging all Allies on board to abandon their ships first. During this battle the Allies lost two destroyers, 14 drifters and one glider while the Austro-Hungarian navy suffered only minor damage (Novaras steam supply pipes were damaged by a shell) and few losses. The Austro-Hungarian navy returned to its bases up north in order to repair and re-supply, and the allies had to rebuild the