While performing a reconnaissance mission in the late afternoon of October 8, 1918, Keesler and his pilot, 1LT Harold W. Riley, were attacked by four enemy fighters. Keesler returned fire and shot down the leader, but Riley lost control of their badly damaged plane. Keesler continued to fend off the attackers with machine gun fire even as it fell. He was wounded six times in the chest and abdomen before the plane crash-landed. Lieutenant Keesler received an additional wound when the enemy fighters strafed them on the ground.