In 1933, Hassell joined the Nazi Party. He was strongly against the Anti-Comintern Pact concluded by Nazi Germany, Fascist Italy and the Empire of Japan in 1937 and favoured instead Western-Christian unity in Europe (he was, in fact, a member of the Order of Saint John, a German Protestant noblemen's association, to which he had been admitted as a Knight of Honor in 1925 and he had been promoted to Knight of Justice in 1933). In 1938, as a result of the Blomberg-Fritsch Affair, Hassell was recalled from his posting as ambassador in Rome by Adolf Hitler but without being cast out of the diplomatic service. Soon after the German attack on Poland on 1 September 1939, Hassell led a delegation to allay Northern European governments' fears of a forthcoming German strike on their countries.