The Pope's own imperial army sacked Rome because he couldn't afford to pay them

In May 1527, a mutinous army of unpaid Imperial troops under Charles V, roughly 14,000 German Landsknechte and 6,000 Spanish soldiers, stormed Rome and sacked it for days, killing 147 of the Pope's Swiss Guard and leaving tens of thousands of civilians dead, wounded or exiled. Pope Clement VII was besieged in Castel Sant'Angelo and taken prisoner. Many historians mark this brutal sack, not any single artwork, as the effective end of the Roman High Renaissance.

— Wikipedia contributors, Sack of Rome — 6 May 1527, part of the War of the League of Cognac