Why was oil important to Japan’s military plans in 1941?

In 1941, petroleum was essential to Japan’s industrial economy and military operations. Following Japan’s occupation of French [Indochina], the United States responded by freezing Japanese assets and imposing a total embargo on oil exports to Japan.12

Because Japan possessed limited oil stocks and lacked alternative supply sources, this embargo functioned as an ultimatum that threatened the viability of the Japanese military and its expansionist goals. To maintain its military-industrial advantage and achieve self-sufficiency, Japan determined it had to seize the oil-producing regions of the [Dutch East Indies]. Consequently, the Japanese high command decided that war against the United States and Great Britain was necessary to secure these vital resources, leading to the decision to eliminate the U.S. Pacific Fleet at Pearl Harbor to clear the way for the conquest of Southeast Asia.13