The Pacific War, sometimes called the Asia–Pacific War, was the theater of World War II that was fought in 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 as far as 19 September 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 invaded Thailand and attacked the British colonies of Malaya, Singapore, and Hong Kong as well as the United States military and naval bases in Hawaii, Wake Island, Guam, and the Philippines.The Pacific War saw the Allies pitted against Japan, the latter aided by Thailand and to a lesser extent by the Axis allies, Germany and Italy. Fighting consisted of some of the largest naval battles in history, and incredibly fierce battles and war crimes across Asia and the Pacific Islands, resulting in immense loss of human life. The war culminated in massive Allied air raids over Japan, and the atomic bombings of Hiroshima and Nagasaki, accompanied by the Soviet Union's declaration of war and invasion of Manchuria and other territories on 9 August 1945, causing the Japanese to announce an intent to surrender on 15 August 1945. The formal surrender of Japan ceremony took place aboard the battleship USS Missouri in Tokyo Bay on 2 September 1945. After the war, Japan lost all rights and titles to its former possessions in Asia and the Pacific, and its sovereignty was limited to the four main home islands and other minor islands as determined by the Allies. Japan's Shinto Emperor relinquished much of his authority and his divine status through the Shinto Directive in order to pave the way for extensive cultural and political reforms.