The Marines were soldiers armed with the best weapons available. They knew when enemies were approaching, their weapons fired 10mm, armour-piercing, explosive rounds and they had an advisor on how to combat the Aliens.
They had an ndependently targeting particle beam phalanx. Tactical smart missiles, phase-plasma pulse rifles, RPGs, sonic electronic ball breakers. They had nukes, knives, sharp sticks...
And they all died.