Why did Tuscany, Parma, and Modena seek union with Piedmont in 1859?

In 1859, the duchies of Tuscany, Parma, and Modena sought union with Piedmont primarily due to a surge of nationalist sentiment and spontaneous uprisings that occurred following the outbreak of the war against Austria.12

The primary drivers for this movement included:

• Nationalist Sentiment: The war against Austria acted as a catalyst for revolutionary movements across central Italy. Local populations expelled their old ruling houses and Austrian garrisons, seeking to align themselves with the only Italian state that maintained independence and initiative: Piedmont.134
• Fear of Revolution: The desire for union was also fueled by a fear of democratic or radical revolutionary outcomes. By seeking annexation to the monarchist-led Piedmont, moderate political leaders—such as Baron Bettino Ricasoli—hoped to stabilize the region under a constitutional framework.12
• Political Strategy: These states aimed to weaken Austrian influence in the peninsula and establish a stronger, unified Italian state.1

Although Napoleon III initially opposed these annexations to prevent the creation of a powerful state on France's border, the persistent popular support for union, confirmed by plebiscites, eventually forced a diplomatic resolution that allowed for their incorporation into the growing Italian kingdom.12