The Sack of Rome
Judith Mason describes how, in February 1525, Francis I of France was defeated and taken prisoner at Pavia by an Imperial army, led by his rebellious subject the Constable of Bourbon, who later launched an attack upon the Holy City.
After the battle of Pavia, a decisive French defeat, there remained only two powers capable of thwarting Imperial ambitions in Italy, the Papacy and Venice. Of these two the more vulnerable was the Papacy, chronically short of money, ruling proverbially disloyal states, which contained factions perpetually at war with each other and with successive Popes. From 1525 onwards Charles V had made use of one of these factions, the Colonna, to keep the Papal States in constant turmoil; and in September 1526 a dress-re-hearsal of the Sack of Rome had been staged when the Colonna attacked Rome, took control and drove the Pope to take refuge in Castel San Angelo. The Colonna alone, however, were not powerful enough to cripple the Papacy; and so a plan was laid for using the Imperial army of the Duke of Bourbon, then garrisoned in Milan, to attack Clement VII, a Medici Pope, either in Florence or in Rome, its ultimate objective being to depose Clement and replace him by a Pope more amenable to the Imperial cause. When, in the last week of January, Bourbon led his army out of Milan, he still did not know whether his attack was to be directed against Florence or against Rome.