The battle takes place at Castelfidardo, a small town in the Marche region of Italy. It is fought between the Sardinian army, acting as the driving force in the war for Italian unification, against the Papal States.
On September 7, Camillo Benso, Prime Minister of Piedmont, sends an ultimatum to the Pope Pius IX demanding that he dismiss his foreign troops. When he fails to do this, 35,000 troops cross the border on September 11, with General Enrico Cialdini advancing along the Adriatic coast and General Enrico Morozzo Della Rocca leading another troop across Umbria. Papal troops are caught by surprise and thrown into confusion. Some of the Papal troops surrender the same day and some retreat to Ancona, which falls on September 29, 1860 after a short siege.
The battle is remembered for being bloody and for the highly disparate numbers of troops — less than 10,000 papal soldiers to 39,000 Sardinians. The papal army is composed of volunteers from many European countries, amongst whom the French and Belgian nationals constitute a Franco-Belgian battalion. Among the French volunteers are a notable number of nobles from western France. After the battle, while consulting the list of dead and wounded members of the papal army, the Sardinian general Cialdini is reported to say in an example of rather black humor, “You would think this was a list of invites for a ball given by Louis XIV!”
The Franco-Belgian, Austrian and Irish battalions later join the Papal Zouaves corps, an infantry regiment of international composition that pledges to aid Pope Pius IX in the protection of the Papacy for the remainder of the Italian unificationist Risorgimento. The battle is commemorated by the Italian ironclad Castelfidardo, built in the 1860s and the 26th Bersaglieri Battalion “Castelfidardo.”