Final answer:
Italian nationalists agreed to unite under Piedmont-Sardinia due to its independent status, the skillful leadership of Victor Emmanuel II and Count Cavour, and the strategic opportunity to create a stronger Italian state free from Austrian control.
Step-by-step explanation:
Italian nationalists agreed to unite Italy under the leadership of Piedmont-Sardinia for several strategic and political reasons. The kingdom of Piedmont-Sardinia, under King Victor Emmanuel II and his prime minister Count Camillo di Cavour, was an independent entity and had retained its autonomy post-Napoleonic era. Cavour, the architect of Italian unification, saw the potential of using Italian nationalism to expand Piedmont-Sardinia's influence and counter Austrian dominance in the region.
Cavour orchestrated an astute international policy by aligning with France during the Crimean War, taking advantage of the disrupted balance of power in Europe. This alliance, cemented with Napoleon III, led to military support against Austria, allowing Piedmont-Sardinia to push the Austrians out of northern Italy. Not long after, the southern regions, galvanized by Giuseppe Garibaldi's leadership, joined the cause, ultimately leading to the declaration of a united Kingdom of Italy under Victor Emmanuel II.
The unification movement, known as the Risorgimento, was driven by widespread Italian nationalism, a collective aspiration for independence, and economic progress. By aligning with Piedmont-Sardinia, which had a capable leadership and a tactical geopolitical position, Italian nationalists saw the practical path towards their goal of a unified, independent Italy.