Hitler withdrew Germany from international agreements and invaded Poland in 1939, starting World War 2 in Europe. As the war escalated, President Roosevelt provided increasing aid to Britain and convinced Congress to support the British war effort. Winston Churchill became Prime Minister of Britain in 1940 and worked closely with Roosevelt to coordinate the Allied response to Germany and Italy. After German U-boats attacked American ships, Roosevelt took steps to increase U.S. involvement in the war, which grew further after the Japanese attack on Pearl Harbor in 1941, bringing the U.S. fully into the war.