In the summer of 1941, Japan had been at war on the mainland of Asia for four years. After amputating Manchuria from China proper in 1931, and recreating it as Manchukuo under a puppet regime, she had plunged into a full-scale war of conquest with China in 1937. But although a Japanese army of well over a million men occupied vast stretches of the Chinese mainland, active hostilities showed no sign of diminishing. Despite the installation of a puppet regime in Nanking, and a campaign of intimidation and brutal reprisals to pacify the conquered areas, the drain of manpower and supplies continued unabated.
Just as today, Japan was wholly dependent on outside sources for the minerals, petroleum, and other raw materials necessary to fuel its economy, which in 1941 was already highly industrialized.