Manchukuo, officially the State of Manchuria prior to 1934 and the Empire of (Great) Manchuria after 1934, was a puppet state of the Empire of Japan in Manchuria from 1932 until 1945. It was founded as a republic in 1932 after the Japanese invasion of Manchuria, and in 1934 it became a constitutional monarchy under the de facto control of Japan. It had limited international recognition.

The area was the homeland of the Manchus, including the emperors of the Qing dynasty. In 1931, Japan seized the region following the Mukden Incident. A pro-Japanese government was installed one year later with Puyi, the last Qing emperor, as the nominal regent and later emperor. Manchukuo's government was dissolved in 1945 after the surrender of Imperial Japan at the end of World War II. The territories claimed by Manchukuo were first seized in the Soviet invasion of Manchuria in August 1945, and then formally transferred to Chinese administration in the following year

Demographically, Manchus formed a minority in Manchukuo; the largest ethnic group was Han Chinese. The Japanese population increased immensely under this period, largely in part due to Japan's settlement efforts to move young, land-poor farmers from the inner islands to colonize new land. By 1945, there were more than a million Japanese settlers. The population of Koreans also increased during this Manchukuo period, and there were also Mongols, Russians, and other minorities. The Mongol regions of western Manchukuo were ruled under a slightly different system in acknowledgment of the Mongolian traditions there. The southern tip of the Liaodong Peninsula (present-day Dalian) continued to be directly ruled by Japan as the Kwantung Leased Territory until the end of World War II

Manchuria was resource-rich, as opposed to Japan. There was a tremendous amount of coal, some sizable oil fields, and plenty of iron, nickel, and other important commodities. But there was not a lot of good land and food, which Japan's growing population needed. This was one reason for the move into China and other places in Asia in 1940, such as Indochina (which is how Vietnam, Laos, and Cambodia were once known). The problem was that the more Japan expanded, the more resources it needed to protect the resources it had gained, and the one resource it was not able to seize in decent quantities was oil. For that, it was dependent on oil from the Dutch East Indies (today's Indonesia) and, more importantly, the United States, which was, at that time, the largest oil producer and exporter on the planet.