> I can't think of a guerrilla war since WWII that hasn't been successful.

Modern Western countries haven't had much of a history of successful insurgencies. Hungary and Czechoslovakia were both invaded by the USSR (along with Warsaw Pact allies), and it didn't result in an insurgency. We shouldn't exclude WWII either, since that was the last time many of Western nations were invaded by outside powers. The resistance movements never seriously threatened Axis rule in Western Europe.

Everyone seems to be expecting a large Afghan style insurgency, but I think it's far from certain that it will happen.