The battle of Castelfidardo (18 September 1860) was the most significant battle during the brief Piedmontese invasion of the Papal States and split the Papal field army into several weak fragments.
Cavour, Prime Minister of Piedmont, had several different motives for his invasion of the Papal States on top of his general desire for Italian unification. The northern parts of the Papal States, in the Romagna and Papal Legations, had been annexed after the Franco-Austrian phase of the war in 1859, but the Pope still ruled a band of land that ran across the Italian peninsula, from Rome in the west across to Umbria and the Marches. The newly annexed areas were thus sandwiched between Papal lands in the south and Austrian lands in Venetia in the north. Cavour was worried that the two powers might unite and try and re-conquer the area.
The situation was further complicated by Garibaldi's successful invasion of Sicily, which began in May 1860. As Garibaldi continued to defeat the forces of the Kingdom of the Two Sicilies it became clear that he would soon cross to the mainland. His first target was Naples, but after that he intended to march north and occupy Rome. The city of Rome was garrisoned by the French, and Napoleon III would almost certainly have intervened if Garibaldi attacked. A successful attack on Rome might have been almost as dangerous for Cavour's hopes of an Italy united under Victor Emmanuel II of Piedmont, as it might have placed power in the hands of more radical elements. Cavour was able to use this second threat to convince Napoleon III that the best way to preserve the independence of Rome and protect against the radicals would be for Piedmont to invade the eastern part of the Papal States and then link up with Garibaldi in Naples. Piedmont would then take political and military control of the war and its aftermath.
On 28 August Cavour's emissaries met Napoleon III at Chambéry. The location was perfect for Cavour's purposes, as it fell within Savoy, an area recently handed over to France in return for the Romagna. Napoleon gave his approval for a Piedmontese campaign in Umbria and promised to keep his troops in Rome and the surrounding provinces. The way to the south was clear.
The Papal States was defended by a reasonably sizable army of around 15,000 men. 6,000 of them were Austrian veterans, most of whom had been garrisoned in the area before the fighting in 1859. There were also several hundred Irish soldiers, and the rest of the army was made up of a mix of nationalities. Most of these men were motivated by religious dedication, so their morale and fighting spirit was better than that of the Neapolitan army fighting and losing to their south. In March 1860 command of this army was given to General Lamoricière, a retired French general and opponent of Napoleon III. The general trend of Papal policy was anti-Napoleon, who was blamed for the loss of the Romagna in 1859.
On 7 September, the same day that Garibaldi entered Naples, Cavour sent an ultimatum to the Pope demanding that he dismissed his foreign troops. The invasion followed on 11 September. 35,000 troops crossed the border in two columns. General Cialdini advanced along the Adriatic coast, while General Della Rocca crossed into Umbria in the upper Tiber valley. General Fanti, the Piedmontese high commander, was with Della Rocca.
General Lamoricière responded to the invasion by moving towards Ancona, where he hoped to meet an Austrian expeditionary force. Lamoricière decided to move east via Tolentinoi and Macerata, then on to Loreto near the coast south of Ancona. He then intended to march along the main road into Ancona, where he would defend the port against the Piedmontese.
General Cialdini learnt of Lamoricière's movement and decided to try and block it. He abandoned the coast road just to the north of Ancona, and moved south to Jesi, before turning east and heading to Osimo (south of Ancona). He was also aiming at Loreton.
On 16 September Cialdini's advanced guard reached Castelfidardo and Crocetto, to the west of Loreto. Half of the Papal Army was already at Loreto, with the rest on the way, but a brief chance to defeat the isolated Piedmontese advance guard was lost. On 17 September the two armies were both reunited.
Cialdini had around 16,500 men, although most were never engaged. He posted then on the northern side of the Musone River, guarding the higher ground around the main road to Ancona. He didn't guard the area closer to the coast as he had been told that the river was un-fordable there.
Lamoricière was on the southern side of the river, at Loreto, which was on a hill above the valley. He could no longer hope to reach Ancona along the main road or with most of his baggage, but there was still a chance that he could use coastal tracks to reach his destination. Lamoricière had around 6,500-8,000 men, the rest of his army having been left in garrisons.
Lamoricière caught Cialdini by surprise. His plan was to send 3,000 men under General Pimodan across a ford east of the main bridge. This force would capture the village of Crocette and the Monte d'Oro, a hill to the north-east, and hold that position while the rest of the Papal army moved along the coast. Pimodan's role was to prevent the main Piedmontese army from moving east to intercept this movement.
Although the Piedmontese were caught by surprise, two companies of Bersaglieri who had been posted near the river fought well and delayed Pimodan. By the time the Bersaglieri had been defeated the Piedmontese had rushed reinforcements to the top of the hill. Cialdini rushed to the scene, and ordered his men to make a bayonet charge down the hill. This threw the Papal troops into some confusion, and a fierce fight broke out close to the river. Pimodan was killed, while Lamoricière was dragged into the fighting, leaving the Papal army without a commander.
The second half of the Papal army was meant to cross the same ford as the first, which was to have been kept clear by the fighting at Monte d'Oro. Instead the second column found itself under fire from guns posted on the hills north of the river, while their route onwards was blocked by the battle. This second column broke and fled without entering the battle. The first column fought on for a little longer, but soon joined the rout. Lamoricière, with a few hundred German speaking troops, attempted to reach Ancona along the beach, but this force was also intercepted. The Germans were forced to surrender, while Lamoricière managed to reach Ancona with only forty-five men.
Half of the Papal army surrendered at Loreto later on the same day. The other half dispersed, although most of the men were taken over the next few days. The Pope had lost his field army, and his remaining troops were pinned down in fortresses. Lamoricière took command of the defence of Ancona, but the siege was short, and the city surrendered on 29 September 1860.