All marines encountered are part of either Bravo Company or Colony defense forces, with those in the former (and to an extent, the latter) under the command of Lt.Col. Robert Burns. Due to Sam's position outside the military hierarchy, Sam may neither give or receive orders from them, a fact that is particularly painful in parts of acts 2 and 5, where Sam is not in the company of his ally, Bravo Company CO Robert Burns.
Late in the game they are encountered as enemies, attempting to attack Russia using the station's weapons, ostensibly under the orders of Burns, though his failure to call them off upon renegading on his previous decision to attack suggests that their orders may have come directly from the POTUS instead. They are somewhat stronger than basic Georgie units in combat, but are overall of little threat.
Whilst many marines will undoubtedly die throughout the game, Sam can limit the number slain, both through battle efficiency (so as to eliminate the enemy before they eliminate his allies) or reviving them directly, and the effect of these actions can be seen in the 'Allies Killed' counter at the end of each mission.
Marines wear far simpler and bulkier armour than Sam, resembling the armour worn by the marines of Starship Troopers. They commonly carry assault rifles, and can also be seen(principally in cut scenes) carrying rocket launchers