After reaching an understanding with Napoleon III of France, Cavour started looking for a pretext for war with Austria. In 1859 rebellion broke out in Lombardy and Venice. Cavour with the help of France succeeded in pushing out the Austrians from Lombardy and merged it with Piedmont.
After this success of Piedmont Napoleon III grew suspicious that ultimately Piedmont may lord over the entire Italy and pose a menace to the French border. He, therefore, made a hurried peace with the Austrian empire, whereby Venetia was left with Austria and an Italian federation under the presidency of the Pope was created. This was a serious set back for Cavour.
After the conclusion of peace by Napoleon III with Austria, Piedmont was also forced to conclude Treaty of Zurich with Austria. Under the treaty the right of Piedmont to rule over Lombardy was accepted but Austria retained her mastery over Venetia. Nice and Savoy were handed over to France. All these developments convinced the Italians that they could hope to liberate their country only through a united action.
The northern States of Italy requested their merger with Piedmont which was duly accomplished after plebiscite. As a result, with the exception of Venetia and Rome the entire northern and central Italy was united and a federal administration was established.