The Pacific War, also known as the Asia-Pacific theater, was the theater of World War II that was fought in Asia and the Pacific Ocean from 1941 to 1945. The war began with a multi-pronged Japanese offensive against British, Dutch, and American colonial possessions in Southeast Asia and the Pacific islands, as well as with the Imperial Japanese Navy's pre-emptive Attack on Pearl Harbor. Japan had been motivated to join the war due to its Tripartite Pact alliance with the other two major Axis Powers, Nazi Germany and Italy, as well as because of its colonial ambitions in the Pacific (especially in the Philippines), and its anger at the United States for its costly oil embargo (which impaired the Japanese war effort in China, and required Japan to acquire alternative oil sources through their invasions of French Indochina and the Dutch East Indies).
The Japanese offensives of late 1941 and early 1942 were mostly successful, with Japan defeating the United States in the Battle of the Philippines, seizing the Dutch East Indies, occupying several British and Commonwealth-ruled Pacific islands, and invading Burma. The United States checked Japan's efforts to establish naval superiority at the Battle of Midway in June 1942, and the Americans went on to counterattack in a series of "island hopping" campaigns, starting with the Battle of Guadalcanal. The second half of 1942 saw the tide turn in favor of the Allies, as the Japanese war machine was halted on all fronts. In 1943, the Americans and Australians began a campaign to liberate New Guinea from the Japanese, but the British, Commonwealth, Chinese, and American forces in Burma were locked in a stalemate against the Japanese; the Japanese continued to score victories against the China on Chinese soil as well. In 1944, however, the Japanese offensive in India and Burma was defeated at Imphal and Kohima, and the Americans recaptured Guam, Tinian, Saipan and several other small islands before defeating the Japanese navy at the Battle of Leyte Gulf and beginning their liberation of the Philippines. By early 1945, the Americans had liberated the Philippine capital of Manila, and American bombers based in the Pacific islands began to launch devastating air raids on the Japanese mainland, including destructive firebombings on cities such as the capital, Tokyo. Meanwhile, American forces invaded Japanese soil when they landed on Iwo Jima and Okinawa, where they were bogged down in hard-fought, bloody battles in the face of desperate Japanese resistance. Iwo Jima fell on 26 March 1945, while Okinawa fell on 21 June; 94% of the 117,000 Japanese troops on Okinawa were killed in action. In China, the Kuomintang retook Henan and Hubei and captured Guangxi, Japan's last stronghold in South China, in August. The British and Commonwealth forces also began their liberation of Borneo, and, by August, the British were planning amphibious landings in occupied Malaya. By this time, however, the United States had decided to force an end to the war with the experimental use of atomic bombs for two bombing raids in Japan. The US dropped "Little Boy" on Hiroshima on 6 August 1945 and "Fat Man" on Nagasaki on 9 August, killing 240,000 people in the two attacks. The Japanese government, which was already considering surrender due to the incendiary bombing campaign, the Allied naval blockade, and the Soviet Union's entry into the war with their invasion of Manchuria, hastily agreed to surrender on 15 August. Emperor Hirohito addressed his nation on the radio, justifying his decision to surrender by saying that continued war would result in the collapse and obliteration of the Japanese nation, as well as the "total extinction of human civilization". On 2 September 1945, the Japanese formally surrendered aboard USS Missouri in Tokyo Bay, and the United States proceeded to occupy the Japanese home islands. The surrender of Japan ended both the Pacific War and World War II, and it left over 4 million Allied soldiers and over 26 million civilians (both figures mostly representing Chinese losses) dead, while Japan lost over 2.5 million soldiers and over 1 million civilians.