Italy was unified when Italian troops entered Rome after the withdrawal of French troops. The Italians stripped all temporal power from Pope Piius IX, whom they imprisoned in the Vatican. Rome became the new capital of unified Italy.

'Two prominent figures in the unification movement were Giuseppe Mazzini, the apostle of Italian unity, and Giuseppe Garibaldi. Among the more conservative constitutional monarchic figures, Count Cavour and Victor Emmanuel II, later the first king of a united Italy, were also important. Mazzini's activity in revolutionary movements caused him to be imprisoned soon after he joined. While in prison, he concluded that Italy could and therefore should be unified and formulated his program for establishing one, free, independent, republican nation with Rome as its capital. After his release in 1831, he went to Marseille, where he organized a new political society called La Giovine Italia ("Young Italy"). The new society, whose motto was "God and the People," sought the unification of Italy. Garibaldi, a native of Nice (then part of the Kingdom of Sardinia), participated in an uprising in Piedmont in 1834, was sentenced to death, and escaped to South America. He spent fourteen years there, taking part in several wars, and returned to Italy in 1848.' 'The Italian Army, commanded by General Raffaele Cadorna, crossed the papal frontier on 11 September and advanced slowly toward Rome, hoping that a peaceful entry could be negotiated. The Italian Army reached the Aurelian Walls on 19 September and placed Rome under a state of siege. Although now convinced of his unavoidable defeat, Pius IX remained intransigent to the bitter end and forced his troops to put up a token resistance. On September 20, after a cannonade of three hours had breached the Aurelian Walls at Porta Pia, the Bersaglieri entered Rome and marched down Via Pia, which was subsequently renamed Via XX Settembre. 49 Italian soldiers and four officers, and 19 papal troops died. Rome and Latium were annexed to the Kingdom of Italy after a plebiscite held on October 9. Initially the Italian government had offered to let the pope keep the Leonine City (the walled part of Rome on the opposite side of the Tiber from the Seven Hills of Rome). But the pope rejected the offer because acceptance would have been an implied endorsement of the legitimacy of the Italian kingdom's rule over his former domain. Pius IX declared himself a prisoner in the Vatican, although he was not actually restrained from coming and going. Rather, being deposed and stripped of much of his former power also removed a measure of personal protection — if he had walked the streets of Rome he might have been in danger from political opponents who had formerly kept their views private. Officially, the capital was not moved from Florence to Rome until July 1871.' Source: http://en.wikipedia.org/wiki/Italian_unification