In response to the emergency of World War II, Hélion returned to France in 1940 and joined the armed forces. Taken prisoner on June 19, 1940, he was held on a prison ship at Stettin an der Oder (now Szczecin, Poland) until February 13, 1942, when he escaped. Four days later he made his way to Paris; by October he was in America, where he spoke on radio and in lecture halls in support of Free France. His book about his experiences, "They Shall Not Have Me", became a best-seller in the United States.