Some historians count the Pacific War and the Second Great European War as to sides of the same conflict, given the ties between the Japanese and the German Empires, and the presence of the United Kingdom, Australia and New Zealand on both conflicts.
Second Sino-Japanese War
While there had been a series of hostilities between Japan and the western powers, the most conventional start of the Pacific War as part of the Global War was 1937 escalation of the Sino-Japanese war of 1931.
This second war between China and Japan begun in 1931 with the Japanese invasion of Manchuria, and became a full scale war after the Marco Polo Bridge Incident on July 7, 1937. In the initial steps, China lost some terrain to Japan but, despite their under-trained military, manage to offer a good resistance to Japan. Despite of this, the western powers did not commit in support of China. However as news of the Japanese massacres in China reached the West, the attitude changed. Australia and the US begun embargo against Japan, and a US volunteer group called the Flying Tigers begun to operate from China with US support.
Japan strikes against the Western powers
As France and the Netherlands felt, Japan attempted to grab French and Dutch possessions in East Asia and the Pacific, but only managed to invade French Indochina in 1940. However as the oil and metal embargo by the US became more severe, the Japanese decided to attack the eastern possessions of the Western Powers.
In December 1941, Japan attacked Pearl Harbor and invaded Dutch West Indies, Hong Kong, Philippines, Thailand, and Malaya. This triggered and official declaration of War by the United States of America, China, the United Kingdom, Australia, and New Zealand.
In the next few months Japan consolidated her power on most Southeast Asia and the Pacific.
In the Battle of Coral Sea, in May 15, 1942, the combined navies of the United States and Australia engaged the Japanese Imperial Navy. The battle expanded for four days and ended with each side losing one large carrier and one destroyer, plus an American oil tanker and a Japanese light carrier, several airplanes, and hundreds of lives.
This battle stopped the Japanese advance and marked the beginning of the Allied counter-attack. A final attack to the Aleutians by the Japanese was stopped by the US Navy, heavily hitting the Japanese Imperial Navy and causing them to lose yet another carrier.
The allies discussed two main strategies: the Island Hoping favored by the Americans, and the reconquest of Burma for linking the Chinese favored by the British.
Finally the US decide to pursue both strategies, supporting the British Commonwealth in re-establishing the link to China and recovering the Pacific Islands.
The Burma Campaign finally laid its fruits by April 1943, when the western allies managed to secure a corridor from India to China over Burma, allowing to ship massive aid to the Chinese resistance. With a majority of Indian born soldiers, the British Army joined the Chinese Kuomintang troops, soon followed by American Lend-Lease and troops.
The original advances were slow, as roads were not prepared, but by December 1943 the tide was clearly favorable to the Chinese, and most of the Yangtze were controlled by the Allies. These positions also helped the Allies to move into Thailand and Indochina. Thailand was freed on late December.
Shanghai was freed on February 1944, close to the liberation of the Philippines, and Formosa was occupied by the Allies by mid March 1944.
With the control of Shanghai, the Philippines and Formosa, the allies managed to cut the supply lines supporting the Japanese in souther China, Indochina, Malaya and the Dutch East Indies, as well as establishing better supply lines from Australia and the USA into China.
With modern western artillery and airplanes, the Chinese moved northwards and Peking was freed on August.
Airfields in Shanghai became the base of operation of American and, after the fall of Germany, British bombers, that established a bombing campaign over Japan.
The strategy of island hopping consisted in invading each island occupied by the Japanese, beginning with those originally belonging to the USA, and then into the Japanese home islands. This strategy was primarily based on the perception that the Asian continent was unavailable for the US to operate, giving how easily Southeast Asia felt, and the stalemate in Burma.
The campaign begun with Guadalcanal, followed with New Guinea and the Solomon islands. But the heavy casualties and losses, plus the success of the Burma campaign, set a hold on the island hopping strategy, and concentrated the efforts into the Philippines and then into Formosa and mainland China.
While the bombing campaign became more dominant, particularly after the liberation of Shanghai and Nanking, the Island Hopping strategy was not completely abandoned. After Formosa, Okinawa and Iwo Jima were occupied, then Guam and the Japanese held Marshal islands.
The Soviet Union enters the Pacific War
In February 1945, the Soviet Union declares war on Japan, soon followed by a massive and successful invasion of Manchukuo and southern Sakhalin. Southern Manchukuo and Korea were also occupied by Kuomintang China, supported by the Western Allies. Most of Korea were in Soviet or Allied control by May 1945.
In April 1945, Soviet leader Joseph Stalin met with the UK Premier Winston Churchill and the American President Henry Wallace at Bari in order to coordinate the defeat of Japan and post-war order. Using Korean airstrips as a a new bombers base (as well as Okinawa and Bond islands, the allies should built up an plan to invade Kyushu, while another more discrete build up in Sakhalin would take place, using mainly experienced British troops from Norway and France.
The Korean assault fleet departed on June 6, 1945, preceded by heavy bombardments on the defenses of Kyushu, and extensive air cover. That night, while destroyers were bombing Kyushu defenses British commandos jumped over Hokkaido in order to destroy the alert and defense system, and a second fleet using no air cover (but covered by submarines) advanced from Sakhalin. In the morning of June 7, a beachhead had been established and the fleet from Korea moved northwards. The defense of Hokkaido was fierce, but the use of heavy bombs helped the allies to hold their ground. By the end of June, the allies controlled extensive areas in Hokkaido and operated airfields from there. The actual invasion of Kyushu begun on June 28.
In July 1945, the Japanese had lost almost all territories outside the Home Islands, and even the sanctuary of the home islands had been violated by invading troops from the United States, the Soviet Union, the British Empire, China, Korea, Australia, New Zealand, Canada, and France (plus volunteers from other parts of the World). The Japanese airspace was controlled by the allies who kept bombing day and night over Tokyo, Yokohama, and Osaka, as well as other targets every few days. US submarines operated with impunity within the Japanese straits and bays.
The Allies had been cautious to hold just enough territory in Kyushu and Hokkaido as they could control, knowing that the Japanese would fight with extreme fierce in defense of their home islands. People were starving. People were dying in the bombings. People were reading and listening to allied propaganda.
The United States had a final ace to play if Japan would not surrender, but was very cautious to play it. Intelligence had discovered that the Soviet Union was also preparing their own ace. Also the British had their own nuclear weapon program, but it seems that the British were way behind. The US supreme command were in heavy internal discussion weather detonating a nuclear bomb in Japan or not. The Japanese had been so badly hit with conventional weapons, that a city-blaster might not mean much while that would give the Russians an idea on how advance the American nuclear program was. On the other hand, the Japanese were not surrendering. It seems they would rather die, even from hunger and mistreatment by their own government, than to allow foreigners to invade the home islands. Also, a detonation would also be a warning to the Russians.
Finally the supreme command decided to wait until they had a pretty good arsenal to be ready to deploy and use after they showed the power in Japan (or wherever it would be).
The winter came, and many more Japanese died from it. In February 1946, Emperor Hirohito demanded the military leadership to step away and to send a white flag to the invading forces. When Tojo arrested the Emperor, most Japanese resistance turned inside in a civil war cognate. The resolution was quick. Admiral Yamamoto arrested Prime Minister Tojo, freed the Emperor and fulfilled the Emperor's wishes. During the cognate, the allies completed the occupation of Kyushu and Hokkaido.
Negotiations continued during a truce. The Japanese wanted a honorable defeat while the allies insisted in an unconditional surrender.
The final agreement was signed on June 23, 1946, by the Emperor Hirohito in representation of Japan, Admiral Nimitz from the United States, and representatives of China, the Soviet Union, and the United Kingdom. The terms were similar to Germany and Austria: demilitarization plus occupation by the US, China, the Soviet Union and the United Kingdom, while the sovereignty were preserved and no occupying power would claim annexation. Only the home islands would be recognized as Japan.