The siege of Ancona (18-29 September 1860) was the last major action during the brief Piedmontese invasion of the Papal States in 1860, and saw the fall of the only port that might have been used by an Austrian expeditionary force, greatly reducing the risk of foreign intervention in the war.
The Piedmontese had invaded the Papal States on 11 September. General Lamoricière, commander of the Papal Army decided to try and reach Ancona, the main Adriatic port still in Papal hands. The only way that the Papal forces could hope to defeat the Piedmontese was with foreign help. The Austrians, who had garrisoned Ancona until 1859, were seen as the most likely to intervene. The Piedmontese also knew this and so half of their army, under General Cialdini, was sent along the coast towards Ancona.
When Cialdini learnt that Lamoricière was heading for Ancona he decided to leave the coast, move around the port and try and block the Papal army to the south. The resulting battle of Castelfidardo (18 September 1860) ended as a crushing Papal defeat. Lamoricière managed to reach Ancona, but only had 45 men with him. The rest of his army was dispersed or captured.
The Piedmontese then turned their attention to Ancona. Cialdini was sent reinforcements from Della Rocca's corps, and he was also supported by the Piedmontese fleet, which provided a fleet of 13 warships under Admiral Count Carlo Pellione di Persano. The fleet also carried the siege train, so Cialdini was soon in a position to begin a regular bombardment of Ancona. This was followed by a series of attacks on the landward fortifications and many of the outworks fell.
The final blow came on 28 September when Persano's fleet began a bombardment of the coastal defences. This ended with the destruction of a powder-magazine. This ended any chances of effective resistance, and on 29 September the city and its garrison of 4,000-6,000 men surrendered. The serious fighting in the Papal States was now over, and the Piedmontese could concentrate to move south to support Garibaldi's campaign in Naples.