After he obtained his bachelor's degree, Tupët-Thomé entered the École supérieure de théologie catholique de Reims. However, he soon decided to join the French Armed Forces and was incorporated into the 8th Zouaves Regiment and stationed in Camp de Châlons. Promoted to sergeant, his regiment was attacked in Lorraine in September 1939 and in Belgium the following year. He participated with his unit in the Dunkirk evacuation from 26 May to 3 June 1940. He was taken prisoner the next day but escaped back to France during his transfer.