Let's say I shark up a list of lawless resolutes, then we hire ourselves out as mercenaries. We get inside our employer's city and loot it, then run off, change our name, and do it again to some other poor sap. Did this ever happen?
I'm not sure about that exact scenario, but in the middle ages a mercenary troop managed to take over a country for themselves after being double crossed by their employer. Look into the Catalan company.