This is fun and all, but the reality is simple:
If (BIG if BTW) there are interstellar space-faring aliens, and they are hostile, we’re TOAST. Game over.
Think what one division of Marines with mechanized armor and helicopter gunships could have done in the Middle Ages. OK, any truly interstellar space-faring race is going to be WAY more advanced compared to us than those Marines would be compare to knights on horseback. WAY MORE.
So let me be the first to swear allegiance to our new alien overlords! /joke
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.