Reaching the rank of major, Leonard commanded a battalion of the 6th Infantry Regiment during Battle of Saint-Mihiel and Meuse-Argonne Offensive.

For his bravery on the battlefield near the village of Romagne-sous-Montfaucon, Major Leonard was promoted to the rank of lieutenant colonel and decorated with the Distinguished Service Cross, Purple Heart French Legion of Honor, Croix de Guerre with Palm and French Fourragère.

[1] His official Distinguished Service Cross citation reads: After the war, Leonard stayed in Europe for occupation duties until October, 1921.

[3] Then he spent some time on the staff of the 2nd Infantry Division under command of Major General James Lawton Collins.

During the summer of 1942 Brigadier General Leonard was put in command of the newly activated 9th Armored Division at Fort Riley, Kansas.