When America officially entered the War, the American Ambulance Service corps was integrated into the U. S. Army Ambulance Corps and Crosby enlisted. During the Battle of Verdun, he was very close to the front, and ferried wounded soldiers from the front lines to rear areas for three days without relief. On November 22, 1917, as Crosby and his best friend, Way "Spud" Spaulding, and another friend, Ben Weeden, were transporting several wounded soldiers to a medical aid station, Crosby's Ambulance 741 was hit by an artillery shell that landed away, sending shrapnel ripping through the vehicle, completely destroying it. Miraculously, Crosby was unhurt, but Spaulding, following close behind in another ambulance, was struck in the chest by shrapnel. Crosby and Weeden were able to transport him to a hospital. After leaving Spaulding at the hospital in Beaulieu and returning to the aid station, Crosby was seen running in circles, lap after lap, without apparent purpose. Crosby declared later that that was the night he changed from a boy to a man. From that moment on, he never feared death. Spaulding was in intensive care for three months and was released from the hospital after six months.