"Semper Fidelis" - The United States Marine Corps Motto
A marine is the typical grunt unit in game, they have random stats and can come with any weapon. They vary greatly in skill, arms and effectiveness, but make up for it in numbers. Since you will get a lot more Marines than, say, Navy Seals, most of your strategies will revolve around them, using special units to last just a bit longer. It's most effective to move these around in teams of 5 or more men when assaulting the bunker and trenches. Remember to never keep them at the edges of map when new wave comes, as they WILL get slaughtered. It's best to place them in defensive positions such as a trench in groups of few men. The bunker is also useful to make marines more efficient killers and to stop them getting killed so fast.
The Marine is the Vietnam equivalent of the MnB2 Soldier.
The Marine Corp was heavily involved in the Vietnam conflict as all the services were, they worked along side the Army and they were mutually supportive.