After the restitution of Alsace-Lorraine to France, Paul Welsch did French military service to become a reserve cavalry captain. In 1939-40, he took  part in the war on the Lorraine front. He distinguished himself there for his conduct in battle and was decorated with the Legion of Honour on October 4, 1940.

He was taken prisoner at the armistice near Saint-Dié: first encampment at Sélestat, then captivity for five days at the Hirth barracks in Strasbourg. From there he was sent to Mainz and then north of Vienna in Austria in Oflag XVII. He produced a series of drawings inspired from these various stays  which he signed "Velche" to erase any Germanic hints in his name. When he exhibited them in Paris at the Grand Palais in August 1944 then, at the end of 1946, at the Salon des Artistes Libérés, he signed the works that were no signed by his real surname.

In autumn 1941 he was transferred to V.a oflag  in Weinsberg. He devoted his time there to drawing the costumes, the sets, the programmes of the drama group formed by the prisoners: Les Caprices de Marianne by Musset, Les 30 millions de Gladiator by Labiche. Released as a French veteran of the old classes, he passed through Stuttgart before being able to reach Paris. He stays most of the rest of the war in the Dordogne, in Génis, in Périgord Vert.