structural:scholar_edition, structural:summarization
Third War of Independence (1866)

In the Austro-Prussian War of 1866, Austria contested with Prussia the position of leadership among the German states. The Kingdom of Italy seized the opportunity to capture Venetia from Austrian rule and allied itself with Prussia. Austria tried to persuade the Italian government to accept Venetia in exchange for non-intervention. However, on 8 April, Italy and Prussia signed an agreement that supported Italy's acquisition of Venetia, and on 20 June Italy issued a declaration of war on Austria. Within the context of Italian unification, the Austro-Prussian war is called the Third Independence War, after the First (1848) and the Second (1859).

Victor Emmanuel hastened to lead an army across the Mincio to the invasion of Venetia, while Garibaldi was to invade the Tyrol with his Hunters of the Alps. The Italian army encountered the Austrians at Custoza on 24 June and suffered a defeat. On 20 July the Regia Marina was defeated in the Battle of Lissa. The following day, Garibaldi's volunteers defeated an Austrian force in the Battle of Bezzecca, and moved toward Trento.

Meanwhile, Prussian Minister President Otto von Bismarck saw that his own ends in the war had been achieved, and signed an armistice with Austria on 27 July. Italy followed, officially laying down its arms on 12 August. Garibaldi was recalled from his successful march and resigned with a brief telegram reading only " Obbedisco " ('I obey').

Prussia's success on the northern front obliged Austria to cede Venetia (present-day Veneto and parts of Friuli) and the city of Mantua (the last remnant of the Quadrilatero). Under the terms of a peace treaty signed in Vienna on 12 October, Emperor Franz Joseph had already agreed to cede Venetia to Napoleon III in exchange for non-intervention in the Austro-Prussian War, and thus Napoleon ceded Venetia to Italy on 19 October, in exchange for the earlier Italian acquiescence to the French annexation of Savoy and Nice.

In the Treaty of Vienna, it was written that the annexation of Venetia would have become effective only after a referendum —taken on 21 and 22 October—to let the Venetian people express their will about being annexed or not to the Kingdom of Italy. Historians suggest that the referendum in Venetia was held under military pressure, as a mere 0.01% of voters (69 out of more than 642,000 ballots) voted against the annexation.

Austrian forces put up some opposition to the invading Italians, to little effect. Victor Emmanuel entered Venice and Venetian land, and performed an act of homage in the Piazza San Marco.