Original: After the First World War, under the National Defense Act on 4 June 1920, Congress reorganized the U.S. land forces by authorizing a Regular Army, a National Guard, and an Organized Reserve (Officers Reserve Corps and Enlisted Reserve Corps) of unrestricted size, which later became the Army Reserve.
Simplified: The National Defence Act of 4 June 1920 authorized a Regular Army, a National Guard, and an Organized Reserve (Officers Reserve Corps and Enlisted Reserve Corps) of unrestricted size.