United States National Guard
The United States National Guard is involved in the grand theft auto series on many occasions.
The National Guard had its own "territory" at their own part of the Los Santos port. There, they stored weaponry. In the level Robbing Uncle Sam, "Ryder" and "CJ" managed to break through the fences and stole the weapons. They were never heared of in the game since.
Liberty City Stories
They were called to investigate the destruction of Fort Staunton, which they reported was due to explosions caused by weapons underground [which was actualy true].