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The Guard traces its heritage back to the state militias, so they've always had some national responsibilities. They were reorganized into the current configuration in the late 19th or early 20th century, to try and avoid some of the petty political squabbles that used to come up when they were called to duty.

Nearly half the U.S. troops on the front lines in WWI were guard units. Guard units made up much of the initial forces deployed in both WWII and Korea.

By Vietnam, however, because of the size of the standing military and for domestic political reasons, something less that 10,000 of the 5,000,000 personnel who served in theater were from the Guard.