Security forces provide force protection and guard special weapons facilities. It is a B billet for second term Marines and Officers and an entry level contract for enlisted 03s.
Mostly either are assigned to a FAST team or to a company that guards a fixed site.
Somewhere a True Believer is training to kill you. He is training with minimum food and water, in the worst of conditions, day or night. The only thing clean on him is his weapon. The True Believer doesn’t care how hard it is. He knows he either wins or he dies. He doesn’t go home at 1600. He knows only his CAUSE.