Italy became a nation-state belatedly—on March 17, 1861 when most of the states of the peninsula were united under king Victor Emmanuel II of the House of Savoy, which ruled over Piedmont. The architects of Italian unification were Count Camillo Benso di Cavour, the Chief Minister of Victor Emmanuel, and Giuseppe Garibaldi, a general and national hero. In 1866 Prussian Prime Minister Otto von Bismarck offered Victor Emmanuel II an alliance with the Kingdom of Prussia in the Austro-Prussian War. In exchange Prussia would allow Italy to annex Austrian-controlled Venice. King Emmanuel agreed to the alliance and the Third Italian War of Independence began. The victory against Austria allowed Italy to annex Venice. The one major obstacle to Italian unity remained Rome.