At the Congress of Paris concluding the Crimean War, Piedmont-Sardinia’s prime minister Camillo Benso di Cavour leveraged the diplomatic forum to draw attention to Austrian dominance in Italy—an early step in internationalizing the unification issue.
Italian nationalist Felice Orsini’s bomb attack in Paris failed to kill Napoleon III but helped reshape Franco-Italian political dynamics, contributing to conditions in which Napoleon III later backed Piedmont-Sardinia against Austria.
Cavour and Napoleon III met secretly at Plombières-les-Bains to discuss a Franco-Piedmontese war aimed at expelling Austria from northern Italy and the prospective postwar territorial settlement (including compensation to France).
In a high-profile diplomatic moment, Napoleon III told the Austrian ambassador that relations between their governments were “not what they should be,” heightening expectations of a coming rupture over Italy.
France and Piedmont-Sardinia finalized a military alliance (kept secret at the time), committing French support if Austria appeared as the aggressor—turning the Plombières understandings into an operational diplomatic framework for war.
Austria issued an ultimatum requiring Piedmont-Sardinia to demobilize; Piedmont’s refusal helped position Austria as the aggressor, activating the Franco-Sardinian alliance logic.
Austrian forces crossed into Piedmont-Sardinia, marking the outbreak of the Second Italian War of Independence and bringing France into the conflict on Piedmont’s side.
The engagement at Montebello was an early battlefield success for French and Piedmontese troops, strengthening allied momentum as larger forces concentrated in Lombardy.
At Magenta, allied forces defeated the Austrians, opening the way to Milan and accelerating political upheaval across northern and central Italy in favor of unification.
The bloody victory at Solferino/San Martino proved decisive militarily but alarmed Napoleon III about the war’s escalating costs and risks, directly setting the stage for a sudden peace initiative.
Napoleon III and Emperor Franz Joseph concluded a preliminary peace (Villafranca), under which Austria ceded Lombardy (with key fortress exceptions) to France for transfer to Piedmont; the compromise triggered Cavour’s resignation in protest.
The settlement was formalized at Zürich via linked treaties: Austria ceded Lombardy to France, and France transferred it to Piedmont-Sardinia, officially closing the war and reshaping the diplomatic landscape for the next steps of unification.
Plombières Agreement and the Diplomacy Leading to the Second War of Independence (1858–1859)
