With little resistance, Japan invaded and conquered Manchuria, a former region in Northeast China, in 1931. Japan claimed that this invasion was a liberation of the Manchus from the Chinese, although the majority of the population was Han Chinese as a result of the large-scale settlement of Chinese in Manchuria in the 19th century. Japan established a puppet regime there called Manchukuo, and in 1936 created a similar puppet state in Inner Mongolia named Mengjiang.
In 1937, Japan invaded China, launching the Second Sino-Japanese War: a conflict between Japan, Mao Zedong's communists, and Chiang Kai-shek's nationalists. On December 13 of that same year, the Nationalist capital of Nanking surrendered to Japanese troops. In the event known as theNanking Massacre, Japanese troops attacked and killed as many as 300,000 people, including civilians. A puppet state was set up in China quickly afterwards, headed by Wang Jingwei. The Second Sino-Japanese War continued into World War II with the Communists and Nationalists in a temporary and uneasy nominal alliance against the Japanese .
In 1938, the Japanese 19th Division entered territory claimed by the Soviet Union, leading to the Battle of Lake Khasan. This incursion was founded in the Japanese belief that the Soviet Union misinterpreted the demarcation of the boundary, as stipulated in the Treaty of Peking, between Imperial Russia and Manchu China. Joseph Stalin ordered a counterstrike, and the use of superior artillery, armor, and air forces nearly annihilated the Japanese forces. On September 15 an armistice was arranged, and two years later, on April 13, 1941, the parties signed aNeutrality Pact, in which the Soviet Union pledged to respect the territorial integrity and inviolability of Manchukuo, while Japan agreed similarly for the Mongolian People's Republic .
On September 27, 1940, Imperial Japan signed the Tripartite Pact with Nazi Germany and Fascist Italy, establishing what would become known as the Axis Powers. The pact called for mutual protection and technological and economic cooperation .
The Second Sino-Japanese War had seen tensions rise between Imperial Japan and the U.S., and American public opinion was generally against Japan. The U.S. placed embargoes on Japan of strategic materials such as scrap metal and oil, which were vitally needed for the war effort. The Japanese were faced with the option of either withdrawing from China and losing face or seizing and securing new sources of raw materials in the resource-rich, European-controlled colonies of South East Asia.
Facing dwindling domestic reserves, the Imperial Japanese Navy made a surprise attack on Pearl Harbor, Oahu, Hawaii, on December 7, 1941, leading to significant losses for the Pacific Fleet of the U.S. The objective of the attack was to incapacitate the U.S. long enough for Japan to establish its Southeast Asian Empire. The U.S. entered the war on the side of the Allies, and Adolf Hitler of Nazi Germany declared war on the U.S., merging the separate conflicts. Following Pearl Harbor, the Japanese launched offensives against Allied forces in South East Asia, with simultaneous attacks on Hong Kong, British Malaya and the Philippines.
Hong Kong surrendered to the Japanese on December 25th, 1941. The Japanese forced the Allies in Malaya to retreat into Singapore, and on February 15, 1942, Singapore fell to the Japanese, causing the largest surrender of British-led military personnel in history. The Japanese military carried out a purge of the Chinese population in Malaya and Singapore, and are believed to have killed tens of thousands of ethnic Chinese. In the Philippines, the Japanese pushed the combined Filipino-American force towards the Bataan peninsula, and January 1942 marked one of the worst defeats suffered by the Americans, leaving over 70,000 American and Filipino prisoners of war in the custody of the Japanese.
The Japanese then seized the key oil production zones of Borneo, Central Java, Malang, Cepu, Sumatra, and Dutch New Guinea of the late Dutch East Indies, defeating the Dutch forces. They were welcomed as liberating heroes by the oppressed Indonesian natives pursuant to their indigenous legends. The Japanese then consolidated their lines of supply through capturing key islands of the Pacific, including Guadalcanal.
In May 1942, the Japanese failed to defeat the Allies at the Battle of the Coral Sea, despite numerical superiority, and in June 1942 they lost a four-carrier task force at the Battle of Midway. Midway was a decisive defeat for the Japanese, and proved to be the turning point of the war. In September 1942 they were defeated by the Australians in New Guinea at the Battle of Milne Bay; further defeats by the Allies at Guadalcanal in September 1942, and New Guinea in 1943 put the Empire of Japan on the defensive for the remainder of the war.