The events that were criticial to the unification of Italian peninsula were:
1858- First, they formed an alliance with France since Austria during the 19th century was getting in the way. Austria took over the provinces of Venetia and Lombardy in the northern part and several other small states. In order to fix the fiasco, Sardinian Prime Minister, Camilio de Cavour, convinced Napoleon III who was the emperor of France at that time, to drive out Austria by waging war against them.
1859- The combined forces of Sardinia and France had freed some northern parts of Italy except Venetia.
1860- Cavour fostered unity over the northern part of Italy and decided to do the same in the south. He helped the rebels of southern Italy and a small army of nationalists under the leadership of Giusippe Garibaldi who by then was the enabler for the army to capture Sicily and move across the Italian mainland marching towards the northern part from there. He freed most of the states in Southern Italy.
1861- In reaching the mainland, volunteers crowded over Garibaldi's banner. An election was conducted and voters had given permission for Garibaldi to unite the areas he conquered.
*** in the same year, the northern and southern part of Italy was unified under the Victor Emmanuel II who was the King of Sardinia. Even with such triumph, there were still territories that still need to be freed.
1866- Venetia became a part of Italy eventually. The Italian forces took over the Papal States. However, despite the unification, there were still many unresolved problems which made Italy enter the 20th century as a poor country.