In the United States, the National Guard is a military force within an individual state, which can become part of the national army if there is a war or emergency. the leader of the Arkansas National Guard. The military reserve units controlled by each state of the United States, equipped by the federal government and subject to the call of either the federal or the state government. the National Guard a military force in each state of the US which can be used when it is needed by the state or the US government