Wikipedia

The Sack of Rome, then part of the Papal States, followed the capture of Rome on 6 May 1527 by the mutinous troops of Charles V, Holy Roman Emperor, during the War of the League of Cognac. Charles V only intended to threaten military action to make Pope Clement VII come to his terms. However, most of the Imperial army (14,000 Germans, including Lutherans, 6,000 Spaniards and some Italians) were largely unpaid. Despite being ordered not to storm the city, they broke into the scarcely defended city and began looting, killing, and holding citizens for ransom without any restraint.[3] Clement VII took refuge in Castel Sant'Angelo after the Swiss Guard were annihilated in a delaying rear guard action; he remained there until a ransom was paid to the pillagers.

Benvenuto Cellini, eyewitness to the events, described the sack in his works. It was not until February 1528 that the spread of a plague and the approach of the League forces under Odet de Foix forced the army to withdraw towards Naples from the city. Rome's population had dropped from 55,000 to 10,000 due to the atrocities, famine, an outbreak of plague, and flight from the city.

https://en.wikipedia.org/wiki/Sack_of_Rome_(1527)