The movement to unite Italy into one cultural and political entity was known as the Risorgimento (literally, "resurgence"). Giuseppe Mazzini and his leading pupil, Giuseppe Garibaldi, failed in their attempt to create an Italy united by democracy. Garibaldi, supported by his legion of Red Shirts-- mostly young Italian democrats who used the 1848 revolutions as a opportunity for democratic uprising--failed in the face of the resurgence of conservative power in Europe. However, it was the aristocratic politician named Camillo di Cavour who finally, using the tools of realpolitik, united Italy under the crown of Sardinia.
"Realpolitik" is the notion that politics must be conducted in terms of the realistic assessment of power and the self-interest of individual nation-states (and the pursuit of those interests by any means, often ruthless and violent ones) and Cavour used it superbly. In 1855, as prime minister of Sardinia, he involved the kingdom on the British and French side of the Crimean War, using the peace conference to give international publicity to the cause of Italian unification. In 1858, he formed an alliance with France, one that included a pledge of military support if necessary, against Austria, Italy's major obstacle to unification. After a planned provocation of Vienna, Austria declared war against Sardinia in 1859 and was easily defeated by the French army. The peace, signed in November 1959 in Zurich, Switzerland, joined Lombardy, a formerly Austrian province, with Sardinia. In return, France received Savoy and Nice from Italy--a small price to pay for paving the way to unification.
Inspired by Cavour's success against Austria, revolutionary assemblies in the central Italian provinces of Tuscany, Parma, Modena, and Romagna voted in favor of unification with Sardinia in the summer of 1859. In the spring of 1860, Garibaldi came out of his self-imposed exile to lead a latter day Red Shirt army, known as the Thousand, in southern Italy. By the end of the year, Garibaldi had liberated Sicily and Naples, which together made up the Kingdom of the Two Sicilies. Cavour, however, worried that Garibaldi, a democrat, was replacing Sardinia, a constitutional monarchy, as the unifier of Italy. To put an end to Garibaldi's offensive, Cavour ordered Sardinian troops into the Papal States and the Kingdom of Naples. After securing important victories in these regions, Cavour organized plebiscites, or popular votes, to annex Naples to Sardinia. Garibaldi, outmaneuvered by the experienced realist Cavour, yielded his territories to Cavour in the name of Italian unification. In 1861, Italy was declared a united nation-state under the Sardinian king Victor Immanuel II.
Reapolitik continued to work for the new Italian nation. When Prussia defeated Austria in a war in 1866, Italy struck a deal with Berlin, forcing Vienna to turn over Venetia. In addition, when France lost a war to Prussia in 1870, Victor Immanuel II took over Rome when French troops left. The entire boot of Italy was united under one crown.
Why did Cavour succeed and Garibaldi fail? Was it really only a matter of speed? If Garibaldi had started his crusade earlier and had time to conquer the Papal State before Cavour sent his troops to do so, would Cavour have been forced to give up his territory in the name of a united Italy? Doubtful. But is speed really the only issue? That, too, is doubtful. It seems that of the two, Cavour alone understood the relationship between national and international events, and was thus able to manipulate foreign policy for his own ends. Garibaldi, a democrat, a warrior, and an anti-Catholic, was without question on the road to conflict with the monarchies of Europe. Cavour, with the added credibility of representing a monarch, blended perfectly with the political situation in Europe at the time.
Cavour was a realist who practice realistic politics. He allied with France when necessary and with France's key enemy, Prussia, was necessary. By keeping the goal in mind, Cavour used international power to achieve his domestic goals. Garibaldi was forced to use his own grassroots strength, empowered by young Italian democrats interested in an idealistic future for their nation. In that manner, it is quite doubtful that Garibaldi would have ever been able to gain the upper hand in Italy, relative to Cavour.