Chamberlain felt betrayed by the Nazi seizure of Czechoslovakia, realized that his policy of appeasement towards Hitler had failed and so began to take a much harder line against Germany. He immediately began to mobilize the British Empire's armed forces to a war footing, and France did the same. Italy saw itself threatened by the British and French fleets and started its own invasion of Albania in April 1939. Although no immediate action followed, Hitler's invasion of Poland on 1 September officially began World War II.