The Pacific War, sometimes called the Asia–Pacific War, was the theater of World War II that was fought in eastern Asia, the Pacific Ocean, the Indian Ocean, and Oceania. It was geographically the largest theater of the war, including the vast Pacific Ocean theater, the South West Pacific theater, the Second Sino-Japanese War, and the Soviet–Japanese War.
The Second Sino-Japanese War between the Empire of Japan and the Republic of China had been in progress since 7 July 1937, with hostilities dating back to 1931 with the Japanese invasion of Manchuria. However, it is more widely accepted that the Pacific War itself began on 7 December (8 December Japanese time) 1941, when the Japanese simultaneously attacked American military bases in Hawaii, Wake Island, Guam, and the Philippines and invaded Thailand and the British colonies of Malaya, Singapore, and Hong Kong.
The Pacific War saw the Allies pitted against Japan, the latter aided by Thailand and to a lesser extent by the Axis powers, Germany and Italy. The Japanese achieved great success in the initial phase of the campaign, but were gradually driven back using an island hopping strategy. The Allies adopted a Europe first stance, giving first priority to defeating Nazi Germany, but still managed to bring to bear the vast industrial might of the United States. The Japanese had great difficulty replacing their losses in ships and aircraft, while American factories and shipyards produced ever increasing numbers of both. Fighting included some of the largest naval battles in history and massive Allied air raids over Japan, as well as the atomic bombings of Hiroshima and Nagasaki.
Japan surrendered unconditionally on 15 August 1945 and was occupied by the Allies. Japan lost its former possessions in Asia and the Pacific and had its sovereignty limited to the four main home islands and other minor islands as determined by the Allies.
In Allied countries during the war, the "Pacific War" was not usually distinguished from World War II, or was known simply as the War against Japan.