The United States National Guard is a unit of aid and defense for the United States. The unit consists of the Army National Guard and the Air National Guard.
In the first F.E.A.R. First Encounter Assault Recon installment, the National Guard is mentioned in the radio new report as the force fighting the "terrorists". This was, of course, a cover to keep people as calm as possible. In actuality, a psychic commander is taking revenge on Armacham Technology Corporation for the tortures committed against his mother.
In F.E.A.R. 2: Project Origin, the National Guard is mentioned on News 22, acting as the primary group setting up shelters and evacuating Fairport after the explosion of the Origin Facility took place. Their activity seems not much as ATC quickly send their forces in attempt to contain the situation. Because the ATC Black Ops they send in suffered heavy losses and lost their leader, a more radical attempt to silence the witnesses is conducted in the next game.
F.E.A.R. 3 
The National Guards are only mentioned briefly in the back story, its implied the ATC influenced the U.S. government and military to halt their operations, then the ATC forces used the evacuation points and shelters to gather and execute the civilians that not influenced by Origin Facility Explosion (Those who did become cultists and cannibals), in orders to keep their projects secret. Cargos with executed citizens are seen in the end of Interval 05.