The National Guard was a reserve component of the United States Army that was tasked with domestic defense and disaster relief in peacetime; although in wartime they were often deployed abroad as front-line soldiers.
Many were dispatched to prepare for the likely event that China would attack American soil with nuclear weapons. They are widely noticed in the Capital Wasteland, where they saw to the aftermath of the Great War. In particular areas, like Washington D.C, they were mobilized to set up an evacuation plan for those in the city and outlying areas.
Fallout 3 is one of the main games which show the presence of the National Guard. The Lone Wanderer can find many National Guard checkpoints around the Capital Wasteland which are usually guarded by an Army robot and contain radiation warning signs alerting people that they near a highly irradiated area, in particular Vault 87. If one travels to Germantown Police HQ they will find a terminal which mentions comments from a female Guardsman present in the area. This is evident as she mentions her sorrow for a fellow member who was sent to the Downtown D.C. area at the time of the Great War, and her anger towards her superiors who have tasked her with organizing farmers into caves somewhere.