Second Sino-Japanese war and World War II

Under the pretext of the Manchurian Incident, Lieutenant Colonel Kanji Ishiwara invaded Inner (Chinese) Manchuria in 1931, an action the Japanese government ratified with the creation of the puppet state of Manchukuo under the last Chinese emperor, Pu Yi. As a result of international condemnation of the incident, Japan resigned from the League of Nations in 1933. After several more similar incidents fueled by an expansionist military, the second Sino-Japanese War began in 1937 after the Marco Polo Bridge Incident.

During the first part of the Shōwa period, according to the Meiji Constitution, the Emperor had the "supreme command of the Army and the Navy" (Article 11). From 1937, Emperor Shōwa became supreme commander of the Imperial General Headquarters, by which the military decisions were made. This ad-hoc body consisted of the chief and vice chief of the Army, the minister of the Army, the chief and vice chief of the Navy, the minister of the Navy, the inspector general of military aviation, and the inspector general of military training.

Having joined the Anti-Comintern Pact in 1936, Japan formed the Axis Pact with Germany and Italy on September 27, 1940. Many Japanese politicians, believed war with the Occident to be inevitable due to inherent cultural differences and Western imperialism. Japanese imperialism, was then justified by the revival of the traditional concept of hakko ichiu, the divine right of the emperor to unite and rule the world.

Japan fought the Soviet Union in 1938 in the Battle of Lake Khasan and in 1939 in the Battle of Khalkhin Gol. Comprehensive defeat of the Japanese by the Soviets led by Zhukov in the latter battle led to the signing of the Soviet–Japanese Neutrality Pact.

Tensions were mounting with the U.S. as a result of public outcry over Japanese aggression and reports of atrocities in China, such as the infamous Nanjing Massacre. In retaliation to the invasion of French Indochina the U.S. began an embargo on such goods as petroleum products and scrap iron. On July 25, 1941, all Japanese assets in the US were frozen. Because Japan's military might, especially the Navy, was dependent on their dwindling oil reserves, this action had the contrary effect of increasing Japan's dependence on and hunger for new acquisitions.

Many civil leaders of Japan, including Prime Minister Konoe Fumimaro, believed a war with America would end in defeat, but felt the concessions demanded by the U.S. would almost certainly relegate Japan from the ranks of the World Powers, leaving it prey to Western collusion. Civil leaders offered political compromises in the form of the Amau Doctrine, dubbed the "Japanese Monroe Doctrine" that would have given the Japanese free rein with regards to war with China. These offers were flatly rejected by U.S. Secretary of State Cordell Hull; the military leaders instead vied for quick military action.