so they actually just kind of switch sides instead of just vanishing. traces back to the days of the Roman Empire, even.
After the infamous Battle of Cannae where Hannibal killed roughly TWENTY PERCENT of Rome's military-aged men, a HUGE chunk of southern Italy defected to the Carthaginian side.
But of course, the small cities in southern Italy that Hannibal was relying on kept switching sides because Rome was beating the living crap out of them constantly for having betrayed the Roman cause.