The Marines would die horribly when they ran out fuel and ammo, which would be pretty quick.
We are talking one hell of a supply line if aliens are invading and starting a shooting war.
More likely they would deal with humans as a pest control issue - engineer a human-specific disease, and let fly. Or hit us a with a bunch rocks at 10% C and clean the survivors.
I’m guessing that I know different Marines than you know. Just a hunch...