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: After the First World War Congress reorganized the U.S. land forces . The National Defense 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 .