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.