The unification of Italy was complete but without Rome and Venetia. In 1866 Italy got an opportunity to acquire Venetia. Chancellor Bismarck of Prussia was making preparations of war against Austria and required Italy’s cooperation in this venture. In April 1866 a treaty was signed between Bismarck and Emmanuel which provided that Venetia would be annexed to Italy in lieu of Italy’s help to Prussia during war against Austria. In June 1866 Prussia declared war against Austria.
Emmanuel's army attacked Venetia from southward in June while Prussia attacked Austria from North. Consequently Austrian forces were divided. Although Italy fought with great courage and bravery. Austria frustrated Italy at several places. On the contrary Prussia defeated her in July 1866 in the battle of Sadowa.The victory of Prussia enabled the unification of Venetia to Italy and by means of referendum Venetia was merged into Italy.