The militia tradition, with its origins in the Minutemen of the Revolutionary War, is enshrined in the U.S. Constitution. By 1898 voluntary military units receiving state support were commonly called the National Guard. Guard units joined the Volunteer Army that had formed to fight the Spanish-American War, providing the majority of over 200,000 volunteers supporting an active army of 60,000. Although regular army units did most of the fighting in Cuba, guard volunteers played a major role in the Philippine Insurrection (1899–1902.) After the war, critics claimed that massive volunteering had overwhelmed the War Department's mobilization effort. As with the regular army, the National Guard became the subject of major postwar legislative reforms.

From 1898 to 1945 debates surrounding National Guard reform reflected a tension between the professional officer corps and citizen-soldier militia that dated to the Revolutionary War. Army officers, many of them West Point graduates, claimed the guard was...