The Japanese economy slowed after Japan entered the Showa Period (1926–1989). To recover from the slump, Japan tried to rule China, which had bounteous resources. The Japanese army stationed in Manchuria caused the so-called Manchurian Incident in 1931. In the following year, “Manchukuo” was established by the Japanese as a puppet state. Subsequently, Japan further invaded the mainland; this led to the outbreak of the Second Sino-Japanese War. In Europe, Germany attacked its neighboring countries and was opposed by the United Kingdom and France, triggering World War II.

Japan subsequently formed a military alliance with Germany and Italy, escalating the conflict with the United States and the United Kingdom, both of which supported China. To resolve the conflict, Japan continued talks with the United States, but the talks broke down. On December 8, 1941, Japan attacked Pearl Harbor, Hawaii. On the same day, in Southeast Asia, Japan invaded the Malay Peninsula, which was then under the rule of the United Kingdom. Japan thus started the Pacific War with the United Kingdom and the United States.

Initially, Japan successively gained control of Southeast Asian territories and South Pacific islands. However, as a result of counterattacks by the United States, which had greater resources than Japan, the tide of the war gradually turned against Japan. In 1944, the United States began air raids (attacks by armed planes on a surface target) directly on mainland Japan. In March 1945, the U.S. military landed on Okinawa, where ground warfare erupted.