 Complete the following phrase:  Despite their fractious relationship, Mitchell and Foulois were of one mind on the necessity of forming an air force to centralize control over tactical aviation. In the St-Mihiel Offensive, commencing September 12, 1918, the American and French offensive against the German salient was supported by 1,481 airplanes directed by Mitchell, totaling 24 Air Service, 58 French Aéronautique Militaire, and three Royal Air Force squadrons in coordinated operations. Observation and pursuit planes supported ground forces, while the other two-thirds of the aerial force bombed and strafed behind enemy lines. Later, during the 