Felice Orsini, a nationalist revolutionary who had participated in the uprisings in Rome in 1848-49, thereafter serving as Mazzini's agent in Switzerland, Hungary, and England, had broken with Mazzini in 1857.

Emotionally disturbed, he had begun to plot the assassination of Napoleon III, impelled by the notion that the emperor's death would trigger in France a revolution that would spread to Italy.

On the night of January 14, 1858, he and two accomplices throw bombs at the carriage of Napoleon and Empress Eugénie as they are going to the opera in Paris; although several persons are killed, the intended victims are unhurt.

Orsini is arrested; he will be executed in March.

Ironically, Orsini's attack reminds Napoleon, who remembers the pro-Italian sympathies of his youth, of his wish “to do something for Italy”.

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.