 
Japan participated in World War II from 1939 to 1945 as a member of the Axis. World War II and the Second Sino-Japanese War encapsulate a significant period in the history of the Empire of Japan, marked by significant military campaigns and geopolitical maneuvers across the Asia-Pacific region. Spanning from the early 1930s to 1945, Japan employed expansionist policies and aggressive military actions, including the invasion of the Republic of China, and the annexation of French Indochina.
In 1941, Japan attempted to improve relations with the United States in order to reopen trade, especially for oil, but was rebuffed. On 8 December, 1941, Japan attacked multiple American and British positions in the Pacific. The Pacific War, a major theater of World War II, further intensified Japan's engagements, leading to significant confrontations with Allied forces in the Pacific Ocean and Southeast Asia. Although initially successful, Japan took significant losses at the Battle of Midway. In addition, Japan met significant setbacks in China. On 6 and 9 August, 1945, Japan was hit by two atomic bombs, while the Soviet Union declared war and invaded Manchuria on 8 August. These events led to the surrender of Japan on 15 August.
During the war, the Japanese committed several war crimes, including attacking neutral countries without a prior declaration of war, massacres and rapes of civilians, the use of comfort women, and biological and chemical warfare and experimentation. In addition, prisoners of war were mistreated, executed, and experimented on.
Prelude.
The Empire of Japan had been expanding its territory since the First Sino-Japanese and the Russo-Japanese War, before World War I through the colonisation of Taiwan and Korea. In 1931, Japan invaded and conquered Manchuria in northeast China. The bordering Chinese territory of Jehol was also taken in 1933, and in 1936, Japan created a similar puppet state in Inner Mongolia.
Japanese invasion of China.
The Second Sino-Japanese War (1937–1945) was a military conflict that was primarily waged between the Republic of China and the Empire of Japan. The war made up the Chinese theater of the wider Pacific Theater of the Second World War. The beginning of the war is conventionally dated to the Marco Polo Bridge Incident on 7 July 1937, when a dispute between Japanese and Chinese troops in Beijing escalated into a full-scale invasion. This full-scale war between the Chinese and the Empire of Japan is often regarded as the beginning of World War II in Asia. (However, according to the Chinese Ministry of Education, it marked only a phase in a 14-year war that began with the 1931 invasion of Manchuria.)
As part of its operations against China, on 22 September 1940 Japan invaded French Indochina. On 27 September it signed the Tripartite Pact with Germany and Italy. War with the U.S. and other Western allies of World War II began with the attack on Pearl Harbor on 7 December 1941. Over the course of seven hours there were coordinated Japanese attacks on the U.S.-held Philippines, Guam and Wake Island, the Dutch Empire in the Dutch East Indies, Thailand and on the British Empire in Borneo, Malaya and Hong Kong. The strategic goals of the offensive were to destroy the U.S. Pacific fleet, capture oil fields in the Dutch East Indies, and maintain their sphere of influence in East Asia. It was also to expand the outer reaches of the Japanese Empire to create a formidable defensive perimeter around newly acquired territory.
China fought Japan with aid from the Soviet Union and the United States. After the Japanese attacks on Malaya and Pearl Harbor in 1941, the war merged with other conflicts which are generally categorized under those conflicts of World War II as a major sector known as the China Burma India Theater. Some scholars consider the European War and the Pacific War to be entirely separate, albeit concurrent wars. Other scholars consider the start of the full-scale Second Sino-Japanese War in 1937 to have been the beginning of World War II. The Second Sino-Japanese War was the largest Asian war in the 20th century. It accounted for the majority of civilian and military casualties in the Pacific War, with between 10 and 25 million Chinese civilians and over 4 million Chinese and Japanese military personnel missing or dying from war-related violence, famine, and other causes. The war has been called "the Asian holocaust".
Decision process by Japanese leaders.
The decision by Japan to attack the United States remains controversial. Study groups in Japan had predicted ultimate disaster in a war between Japan and the U.S., and the Japanese economy was already straining to keep up with the demands of the War with China. However, the U.S. had placed an oil embargo on Japan and Japan felt that the United States' demands of unconditional withdrawal from China and non-aggression pacts with other Pacific powers were unacceptable. Facing an oil embargo by the United States as well as dwindling domestic reserves, the Japanese government decided to execute a plan developed by the military branch largely led by Osami Nagano and Isoroku Yamamoto to bomb the United States naval base in Hawaii, thereby bringing the United States to World War II on the side of the Allies. On 4 September 1941, the Japanese Third Konoe Cabinet (Prime Minister Fumimaro Konoe) met to consider the war plans prepared by Imperial General Headquarters, and decided:
Our Empire, for the purpose of self-defense and self-preservation, will complete preparations for war ... [and is] ... resolved to go to war with the United States, Great Britain, and the Netherlands if necessary. Our Empire will concurrently take all possible diplomatic measures vis-a-vis the United States and Great Britain, and thereby endeavor to obtain our objectives ... In the event that there is no prospect of our demands being met by the first ten days of October through the diplomatic negotiations mentioned above, we will immediately decide to commence hostilities against the United States, Britain and the Netherlands.
Vice Admiral Isoroku Yamamoto, the chief architect of the attack on Pearl Harbor, had strong misgivings about war with the United States. Yamamoto had spent time in the United States during his youth when he studied as a language student at Harvard University (1919–1921) and later served as assistant naval attaché in Washington, D.C. Understanding the inherent dangers of war with the United States, Yamamoto warned his fellow countrymen: "We can run wild for six months or maybe a year, but after that, I have utterly no confidence".
Origin of conflict.
Since early 1941 the United States and Japan had been engaged in negotiations in an attempt to improve their strained relations and end the war in China. During these negotiations, Japan advanced a number of proposals which were dismissed by the Americans as inadequate. At the same time the United States, the United Kingdom, and the Netherlands engaged in secret discussions for the joint defense of their territories, in the event of a Japanese attack against any of them. Roosevelt reinforced the Philippines (an American protectorate scheduled for independence in 1946) and warned Japan that the United States would react to Japanese attacks against any "neighboring countries".
Frustrated at the lack of progress and feeling the pinch of the American–British–Dutch sanctions, Japan prepared for war. On 20 November, a new government under Prime Minister Hideki Tojo presented an interim proposal as its final offer. It called for the end of American aid to China and for lifting the embargo on the supply of oil and other resources to Japan. In exchange, Japan promised not to launch any attacks in Southeast Asia and to withdraw its forces from southern Indochina. The American counter-proposal of 26 November required that Japan evacuate all of China without conditions and conclude non-aggression pacts with all Pacific powers. That meant Japan was essentially forced to choose between abandoning its ambitions in China, or seizing the natural resources it needed in the Dutch East Indies by force; the Japanese military did not consider the former an option, and many officers considered the oil embargo an unspoken declaration of war.
Japan planned to rapidly seize European colonies in Asia to create a large defensive perimeter stretching into the Central Pacific. The Japanese would then be free to exploit the resources of Southeast Asia while exhausting the over-stretched Allies by fighting a defensive war. To prevent American intervention while securing the perimeter, it was further planned to neutralize the United States Pacific Fleet and the American military presence in the Philippines from the outset.
Japanese offensives (1941–1942).
On 7 December 1941 (8 December in Asian time zones), Japan attacked British and American holdings with near-simultaneous offensives against Southeast Asia and the Central Pacific. These included an attack on the American fleets at Pearl Harbor and the Philippines, Guam, Wake Island, landings in Malaya, Thailand and the Battle of Hong Kong.
The Imperial Japanese Navy made its surprise attack on Pearl Harbor, Oahu, Hawaii Territory, on Sunday morning, 7 December 1941. The Pacific Fleet of the United States Navy and its defending Army Air Forces and Marine air forces sustained significant losses. The primary objective of the attack was to incapacitate the United States long enough for Japan to establish its long-planned Southeast Asian empire and defensible buffer zones. However, as Admiral Yamamoto feared, the attack produced little lasting damage to the US Navy with priority targets like the Pacific Fleet's three aircraft carriers out at sea and vital shore facilities, whose destruction could have destroyed the fleet on their own, were ignored. Of more serious consequences, the U.S. public saw the attack as a barbaric and treacherous act and rallied against the Empire of Japan. 
The Japanese invasion of Thailand led to Thailand's decision to ally itself with Japan and the other Japanese attacks led the United States, United Kingdom, China, Australia, and several other states to formally declare war on Japan, whereas the Soviet Union, being heavily involved in large-scale hostilities with European Axis countries, maintained its neutrality agreement with Japan. Germany, followed by the other Axis states, declared war on the United States in solidarity with Japan, citing as justification the American attacks on German war vessels that had been ordered by Roosevelt.
The United States entered the European Theater and Pacific Theater in full force. Four days later, Adolf Hitler of Germany, and Benito Mussolini of Italy declared war on the United States, merging the separate conflicts. Following the attack on Pearl Harbor, the Japanese launched offensives against Allied forces in East and Southeast Asia, with simultaneous attacks on British Hong Kong, Thailand, British Malaya, Dutch East Indies, Guam, Wake Island, Gilbert Islands, Borneo and the Philippines.
By 1942, the Japanese Empire had launched offensives in Dutch East Indies, New Guinea, Singapore, Burma, Yunnan and India, the Solomons, Timor, Aleutian Islands, Christmas Island and the Andaman Islands.
By the time World War II was in full swing, Japan had the most interest in using biological warfare. Japan's Air Force dropped massive amounts of ceramic bombs filled with bubonic plague-infested fleas in Ningbo, China. These attacks would eventually lead to thousands of deaths years after the war would end. In Japan's relentless and indiscriminate research methods on biological warfare, they poisoned more than 1,000 Chinese village wells to study cholera and typhus outbreaks. These diseases are caused by bacteria that with today's technology could potentially be weaponized.
South-East Asia.
The South-East Asian campaign was preceded by years of propaganda and espionage activities carried out in the region by the Japanese Empire. The Japanese espoused their vision of a Greater Asian Co-Prosperity Sphere, and an Asia for Asians to the people of Southeast Asia, who had lived under European rule for generations. As a result, many inhabitants in some of the colonies (particularly Indonesia) actually sided with the Japanese invaders for anti-colonial reasons. However, the ethnic Chinese, who had witnessed the effects of Japanese occupation in their homeland, did not side with the Japanese.
Hong Kong surrendered to the Japanese on December 25. In Malaya the Japanese overwhelmed an Allied army composed of British, Indian, Australian and Malay forces. The Japanese were quickly able to advance down the Malay Peninsula, forcing the Allied forces to retreat towards Singapore. The Allies lacked air cover and tanks; the Japanese had air supremacy. The sinking of HMS "Prince of Wales" and HMS "Repulse" on December 10, 1941, led to the east coast of Malaya being exposed to Japanese landings and the elimination of