Brereton left the 12th Aero Squadron on July 1, was promoted to major on July 2 and three days later became Air Service Officer to the I Corps. When the U.S. 2nd Division attacked Chateau-Thierry in mid-July, Brereton flew the first artillery adjustment mission near Vaux with his old command. Chateau-Thierry brought him to the attention of Brig. Gen. Billy Mitchell, who assigned him command of the Corps Observation Wing on August 28, supervising the observation groups of three corps, one army, and a French group, in preparation for the St-Mihiel Offensive. On September 12, 1918, while flying a troops-in-contact observation mission on the first day of the offensive, Brereton became involved in an air-to-air combat over Thiaucourt that earned him the Distinguished Service Cross