This chart is designed to show changes in the levels of military personnel strength in the United States since World War II. The X-axis is years, from 1950 forward; the Y-axis is the number of military personnel, in thousands. In the chart, the four lines represent the Army, Navy, Marine Corps, and Air Force. Troop strength (particularly the Army) peaked in the Korean War (1952) and the Vietnam War (1968), followed by a slow but constant decrease since the early 1970s, attributable to the expiration in 1973 of the Military Service Act of 1967, ending the draft and creating an all volunteer military. The Navy and Marine Corps also experienced constant decreases in the number of personnel, while the number in the Air Force has changed little.