Commanding the 326th Tank Battalion, Brett, now a major, led the first major American tank attack of World War I at the Battle of Saint-Mihiel in September 1918 and, following Colonel George S. Patton's wounding, took command of the 1st Tank Brigade in his absence. Brett received the Distinguished Service Cross for his role in the battle. The Great War came to an end soon after, on November 11, 1918. In addition to the Distinguished Service Cross, his other decorations from the war were: Silver Star with Oak Leaf Cluster, Purple Heart with two Oak Leaf Clusters, French Croix de guerre 1914–1918 with palm and Officer of the Legion of Honour.