after the war, national guard aviation was placed on a permanent basis over the initial opposition of the army's general staff. in 1920, the militia bureau and the army air service agreed on a plan for re-organizing national guard aviation units. on 17 january 1921, the 109th observation squadron of the minnesota national guard (1921–1941) became the first post world war i air unit to receive federal recognition. during the interwar period, 29 observation squadrons were established. they were either integral elements of national guard infantry divisions or assigned to army corps aviation.