On September 18, 1931, Japanese soldiers staged a bomb attack on the South Manchurian Railway, a minimally damaging incident intended to blame the Chinese and justify military action. This event marked Japan’s aggressive expansion, which was driven by the need for raw materials and security against Soviet communism and Chinese nationalism.

In the 1920s, Japan had maintained a cooperative relationship with Western powers, defusing tensions at the Washington Naval Conference. However, the Great Depression and a rise in nationalism led to Japan’s imperial ambitions. Military dominance in the government and a focus on self-sufficiency in resources— particularly oil—shaped Japan’s aggressive policies. Japan’s limited oil resources, which were primarily imported from the United States and Dutch East Indies, made control of these regions vital.

The Japanese Army and Navy disagreed on strategy; the Army prioritized northern expansion, while the Navy aimed at Southeast Asia for resources. Both, however, supported Japan’s dominance in Asia, and this dynamic was known as the Greater East Asia Co-Prosperity Sphere. Japan’s moves in Manchuria led to the establishment of the puppet state of Manchukuo, and international condemnation from the League of Nations did not deter Japan, which exited the League and continued its path.

The Japanese government then enacted the Petroleum Industry Law in 1934, controlling oil imports and market shares to build domestic refining capacity and to prepare for war.