Cavour arranges with Emperor Napoleon III for French military intervention in the event of Austrian aggression against Piedmont at a secret conference held at Plombières, France, in July 1858.

Cavour's goal is the complete expulsion of Austrian troops from the peninsula.

Orsini's attempted assassination of Napoleon III in January 1858 had paradoxically opened an avenue of diplomacy between France and Piedmont.

While in jail awaiting trial, Orsini had written a public letter to the Emperor of the French, ending with, "Remember that, so long as Italy is not independent, the peace of Europe and Your Majesty is but an empty dream... Set my country free, and the blessings of twenty-five million people will follow you everywhere and forever."

Orsini had still been executed, but Napoleon III had begun to explore the possibility of joint operation with Piedmont against Austria.

Cavour and Napoleon meet secretly in July 1858 at Plombières-les-Bains, and the two agree that Piedmont will attempt to provoke war with the Duchy of Modena, obliging Austria to enter, and France will then aid Piedmont.

In return, Cavour reluctantly agrees to cede Savoy (the seat of the Piedmontese royal family) and the County of Nice to France, and also arranges a royal marriage between Princess Clotilde and Prince Napoleon, surprisingly without Victor Emmanuel's consent.

In the same year, Cavour sends his cousin, the famous beauty, photographic artist, and secret agent Virginia Oldoïni, to further the interests of Italian unification with the emperor by whatever means possible, and by all accounts she succeeds, famously becoming the mistress of Napoleon.