Australia's historical ties with Britain meant that with the commencement of World War II her armies were sent overseas to contribute to battles in Europe. Fear from the north was so understated that at the outbreak of open warfare with Japan, Australia itself was extremely vulnerable to invasion (possible invasion plans were considered by the Japanese high command, though there was strong opposition). Australia's policy became based entirely on domestic defense following the attacks on Pearl Harbor and British assets in the South Pacific. Defying strong British opposition, Australian Prime Minister John Curtin recalled most troops from the European conflict for the defense of the nation.