If we’re comparing to human history, the Aztecs weren’t singlehandedly defeated by the Spanish. The conquistadors had plenty of assistance of other nations that wanted to see the Aztecs gone. We all know what happened afterwards, but even if you went back in time you wouldn’t convince a single Tlaxcala warrior that their newfound awesome ally against their sworn enemy was actually the “bad guy” - until much later, that is.

So, if the aliens started an attack against whoever the superpower happens to be, they would have plenty of assistance as well. It’s human nature.