The major reason that Piedmont was able to do this was because it was led by Count Camillo Cavour. Cavour’s intelligent policies allowed the kingdom to expand its power, thus starting the unification of Italy.
Cavour had three main strategies for taking control of most of what is now Italy. First, Cavour used nationalist ideas to try to get more people in other Italian regions to identify with one another as Italians. He supported the Nationalist Society and that group established branches in other Italian states. The society pushed for unification with Piedmont (which was richer than other states and was also more independent from outside influence) as the leading part of Italy. Second, Cavour used foreign alliances to help his cause. He allied Piedmont with France, offering France the chance to defeat Austria (which controlled parts of Italy). This gave Piedmont military help from France in its fight against Austria. Finally, Cavour used military force. He used his army to help conquer much of the South and to convince Garibaldi (Southern independence fighter) to accept the idea of a more united Italy dominated by Piedmont.
Through these strategies, Cavour was able to extend Piedmont’s power over much of Italy. He was the main factor that allowed Piedmont to expand its control.