The 503rd Infantry Regiment and the 2nd Infantry Division are both formations of what military branch
2nd Infantry Division (United States) The 2nd Infantry Division ("Indianhead"; "2ID," "2nd ID", or "Second D") is a formation of the United States Army. Its current primary mission is the defense of South Korea in the initial stages of an invasion from North Korea until other American units can arrive. There are approximately 17,000 soldiers in the 2nd Infantry Division, with 10,000 of them stationed in South Korea, accounting for about 35% of the United States Forces Korea personnel.
143rd Infantry Regiment (United States) The 143rd Infantry Regiment is an airborne infantry formation in the Army National Guard and has one battalion active under the 173rd Airborne Brigade Combat Team as part of the Associated Unit Program, aligning active and reserve units with one another for training and deployment.