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Why did France join Piedmont in the war against Austria in 1859?

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France joined Piedmont in the war against Austria in 1859 primarily as a result of a secret agreement between French Emperor Napoleon III and the Piedmontese prime minister, Count Cavour.

At a secret meeting in Plombières in July 1858, Cavour successfully persuaded Napoleon III to support an anti-Austrian policy. Cavour’s strategic goal was to expel Austrian troops from the Italian peninsula, while Napoleon III sought to reestablish France as the leading power on the continent by replacing Austrian domination with French influence. In exchange for this military intervention, Piedmont agreed to cede the territories of Savoy and Nice to France.123

The alliance was formally sealed in January 1859. The conflict was triggered when Austria, viewing the war as inevitable, issued an ultimatum demanding Piedmontese demobilization. When Piedmont rejected this demand, Austria declared war, and France honored its commitment to intervene on Piedmont's behalf.123