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.
31st Infantry Regiment (United States) The 31st Infantry Regiment ("Polar Bears") of the United States Army was formed on 13 August 1916, and was part of USAFFE's Philippine Division during World War II. The unit is rare in that it was formed and has spent most of its life on non-American soil.