In 1917 the regiment became part of the 2nd Infantry Division, with which it served during World War I, World War II, and the Korean War. Reorganized as a parent regiment during the late 1950s as the United States Army adapted its organization to the Cold War, its 4th Battalion served with the 25th Infantry Division in the Vietnam War. The 9th's 1st, 2nd, and 3rd Battalions served in the 1989–1990 United States invasion of Panama, Operation Just Cause, with the 7th Infantry Division (Light). Its 1st and 4th Battalions fought in the Iraq War; the 4th Battalion is, , the only remaining active battalion of the regiment, stationed at Fort Carson with the 1st Stryker Brigade Combat Team, 4th Infantry Division.