The National Guard was a part of the Armed Forces of the United States of America on Earth. The National Guard had an Army component and a Air Force component. Each member state of the United States had National Guard forces answerable both to the Governor of that State, for emergencies such as a riots or natural disasters, and to the President of the United States of America as a reserve force if there was a need for such personnel for Federal service.
In 1970, Ohio National Guard troops killed four and injured nine at Kent State University during a protest against the Vietnam War. The attention to this tragic event put a great deal of negative attention on the Guard, requiring them to exercise greater restraint in similar subsequent situations. National Guard troops were present at another protest at Illinois State University later in 1970, and nearly opened fire after one of the troops was hit by a thrown bottle. Fortunately, further violence was halted through the intervention of protest organizer Curtis A. Wellborn. (TOS - Star Trek: Assignment: Earth comic: "My Name Is Legion")
During the Breen attack on Earth during the Dominion War, the California National Guard was ordered into San Francisco to prepare for possible enemy invasion and to assist survivors. (ST short story: "Eleven Hours Out")