National Committee for a Free Germany
On 1 January 1943, Riedel was promoted to "Major" (major). One month later, on 28 January, Riedel, who had been injured by artillery shell splinter on his left side, was taken prisoner of war by Soviet forces at the field hospital of the 71st Infantry Division during the Battle of Stalingrad. In 1944, he joined the National Committee for a Free Germany ().