The American oil embargo exposed Japan's dependence on imported fuel. The United States had supplied 93 percent of Japan's oil in 1940. Navy Chief of Staff Nagano warned Hirohito that the stockpiles would be exhausted within two years. He delivered the warning to the emperor because the chiefs of staff reported directly to Hirohito rather than to the prime minister.
Notable Point · 57:27-58:19
Nagano's oil warning and direct access to the emperor
The navy's warning about two years of oil reserves went directly to Hirohito through the military command system.