American involvement in WWII started with the Japanese attack of Pearl Harbor on 7 December 1941. During the preceding two years of the war, the United States had maintained formal neutrality while supplying Britain, the Soviet Union, and China with war material through the Lend-Lease Act. Whilst the US priority was the defeat of Germany in Europe, the war against Japan in the Pacific was more urgent after the sinking of the main battleship fleet at Pearl Harbor. The Imperial Japanese Navy had the advantage and had gained a lot of ground but in June 1942 its main carriers were sunk during the Battle of Midway, and the Americans seized the initiative. The Pacific War became one of island hopping and the army steadily advanced across New Guinea to the Philippines, with plans to invade the Japanese home islands in late 1945. Strategic bombing destroyed all the major Japanese cities and the US captured Okinawa in spring 1945. With the atomic bombings of Hiroshima and Nagasaki and an invasion of the home islands imminent, Japan surrendered. The war in Europe initially involved supplying aid to the allies until the US could ready an invasion force. US troops were first tested in the North African campaign and then employed more significantly with British forces in Italy in 1943–45. Finally the main invasion of France took place in June 1944, while the US Air Force and British RAF engaged in the bombardment of German cities, transportation links and synthetic oil plants.