Aero Squadrons were the designation of the first United States Army aviation units until the end of World War I. These units consisted of combat flying, training, ground support, construction and other components of the Air Service. After World War I ended, the majority of these squadrons were demobilized. Some however were retained during the interwar period of the 1920s and 1930s, and served in all theaters of operation during World War II. Today, the oldest squadrons in the United States Air Force and Air National Guard can trace their lineage back to the original Aero Squadrons of World War I.