How can I put this...
Imagine in one side you have tribal cavesmen. As much as you want. And heck let's throw them any number of beasts of war you want at their side.
And in the other you have a modern combat force with bombers, battleships, combat helicopters, artillery, biological weapons, ect.
Short of you bringing enough cavesmen that you can cover the surface of the planet with enough corpses to drench the oceans and consume all the available oxygen, no, the cavesmen are not gonna win no matter how many you bring.
For a recent example, review the butchering of most american tribes (and yes I'm including south, north and middle here, whole american continent) by the colonizers. Sure the tribes did manage to score some victories here and there... But they then were still utterly crushed no matter how you look at it, despite having massive numerical advantage in the begginning.
But of course in a game, people want their groups of dudes wavering pointy sticks, regardless of being the middle ages or the grimdarkness of the future.