The Air Service replaced its wartime structure with the formation of six permanent groups in 1919, four of which were based in the United States and two overseas. The first of the new groups, the Army Surveillance Group, was organized in July to direct the operations of three squadrons patrolling the border with Mexico, where revolution had broken out, from Brownsville, Texas to Nogales, Arizona. In addition, the 1st Day Bombardment Group was formed to control four bombardment squadrons at Kelly, while the 1st Pursuit Group of four pursuit squadrons relocated from Selfridge Field, Michigan, to add their weight to the effort. Collectively the three groups (the entire combat strength of the Air Service in the continental United States) comprised the "1st Wing". In January 1920 only the surveillance group continued the patrols, which gradually diminished until June 1921 when they ceased entirely.