 After returning from Europe, Knapp was assigned to the 96th Aero Squadron, charged with aerially patrolling the US-Mexican border after a Mexican cross-border raid on El Paso, Texas in 1919.In 1921, he was given command of Flight "A" of 12th Aero Squadron at Camp Stephen B.Little in Nogales, Arizona, where for the next two years he patrolled the Arizona border.