The 509th Infantry Regiment is an airborne infantry regiment of the United States Army. Previously titled the 509th Parachute Infantry Regiment(509th PIR) was the first parachute infantry unit of the U.S. Army to make a combat jump during World War II. Currently its 1st and 3rd battalions are active. The 1st Battalion serves as the Opposing Force (OPFOR) at the Army's Joint Readiness Training Center at Fort Polk, Louisiana. The 3rd Battalion is assigned to the 4th Brigade Combat Team (Airborne), 25th Infantry Division, at Fort Richardson, Alaska.