But that's how we agree... They don't seem to be all powerful.
Right from the beginning, they were being incredibly economical by only attacking the largest groups of human beings, and then picking a slightly smaller number which had been previously allowed to run free because it wasn't worth their time, and then attack that.
Imagine if the Jews running from the the NAZIs, conquered France, throwing the French at the German war machine, then crossed the channel subdued Britain and focussing it's resources and slave labour to a defensible perimeter.
It doesn't matter if the Esepheni are someone elses punk being forced into doing horrible things to survive, they crossed a line.