The Central Powers launched a crushing offensive on 26 October 1917, spearheaded by the Germans, and achieved a victory at Caporetto . The Italian Army was routed and retreated more than 100 kilometres  to reorganise, stabilising the front at the Piave River. Since the Italian Army had suffered heavy losses in the Battle of Caporetto, the Italian Government ordered conscription of the so-called '99 Boys : all males born in 1899 and prior, who were 18 years old or older. In 1918, the Austro-Hungarians failed to break through in a series of battles on the Piave and were finally decisively defeated in the Battle of Vittorio Veneto in October. On 1 November, the Italian Navy destroyed much of the Austro-Hungarian fleet stationed in Pula, preventing it from being handed over to the new State of Slovenes, Croats and Serbs. On 3 November, the Italians invaded Trieste from the sea. On the same day, the Armistice of Villa Giusti was signed. By mid-November 1918, the Italian military occupied the entire former Austrian Littoral and had seized control of the portion of Dalmatia that had been guaranteed to Italy by the London Pact. By the end of hostilities in November 1918, Admiral Enrico Millo declared himself Italy's Governor of Dalmatia. Austria-Hungary surrendered on 11 November 1918.

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Which happened second, the Italian Navy destroyed much of the Austro-Hungarian fleet or the Italians invaded Trieste?