Without warning or even declaring war, Japan attacked the US naval fleet at Pearl Harbor, Hawaii, on 7 December 1941. Japan's military government, which had already occupied Manchuria, Korea, coastal China, and French Indo-China, needed access to raw materials, particularly oil, to continue its imperialist ambitions in the Pacific. A preemptive strike on the US Navy, it was hoped, would allow Japan to dominate this theatre of the Second World War (1939-45).
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/2953/why-did-japan-attack-pearl-harbor/.