Completing unification: Venetia, Rome, and the German Empire (1866-1871)

Italy's alliance with Prussia in 1866 paid off despite Italian military failure: although Italian forces were defeated on land at Custoza (24 June 1866) and at sea by the Austrian fleet at Lissa (20 July 1866), Prussia's victory at Sadowa forced Austria to cede Venetia, which passed to Italy through French mediation and was ratified by an overwhelming plebiscite in October 1866. This left only Rome and Lazio, still under papal sovereignty and protected by a French garrison, outside the Kingdom of Italy. The outbreak of the Franco-Prussian War on 19 July 1870 forced Napoleon III to withdraw that garrison to reinforce the front against Prussia, and after the French collapse at Sedan the Italian army breached Rome's walls near the Porta Pia on 20 September 1870 against only token papal resistance. A plebiscite on 2 October 1870 ratified the annexation, and Rome was proclaimed the capital of the Kingdom of Italy in 1871, completing the territorial unification begun in 1859, though "unredeemed" Italian-speaking territories such as Trento and Trieste would remain under Austrian rule until 1918.