World War II in Asia was preceded by a period of expansion by the militarist regime in Japan. Japanese expansion began in China with the invasion and occupation of Manchuria in 1931. When the League of Nations refused to recognize the puppet state set up there in the wake of the invasion, Japan withdrew from the organization. The Soviet Union followed with a brief invasion of China in 1934, an event that drove Japan to join the Anti-Comintern Pact with Hitler's Germany.
In 1937, Japan launched a full-scale invasion of China, beginning a long and brutal struggle that led to enormous loss of life on the part of the Chinese, notably at Nanking. Early fighting also broke out between the Soviet Union and Japan in Mongolia, but the Japanese managed to withdraw from that conflict and focused on Southeast Asia. They gained control of both Indochina and the Dutch East Indies, prompting the United States to announce a complete oil embargo against Japan.
Many Japanese officers in government hoped to create what they called a Greater East Asia Coprosperity Sphere free of European influence and subject to Japanese hegemony. They thus regarded this embargo as a provocative act, and, in December of 1941, they launched a massive offensive against British colonies in Hong Kong, Malaya and elsewhere in Southeast Asia, and most notoriously, against the US Pacific Fleet at Pearl Harbor. This action began the war for the United States, but it was an extension of what had been almost a five-year conflict in Asia.