In September 1940 Japan coerced Vichy France, the puppet government set up in southern France by the victorious Nazis, into giving up northern Indochina. The United States retaliated by prohibiting the exportation of steel, scrap iron and aviation gasoline to Japan. On July 23, 1941, Japan occupied southern Indochina. Two days later, the United States, Britain and the Netherlands froze all Japanese financial assets. The effect was to prevent Japan from purchasing oil, which would, in time, cripple its army and make its navy and air force completely useless. The trade embargo convinced the Japanese government that armed struggle against the United States might be necessary to ensure the economic survival of Japan.