What caused Pearl Harbor?
The unprovoked attack of the US Naval Fleet in Pearl Harbor, Hawaii was due to the Japanese motivation to expand their Imperial rule; in order to take over Asian Pacific islands and nations that had the natural resources and oil they desperately needed. The main island of Japan (Honshu) does not have metal ores or oil nor do the smaller islands.
In the middle 1930s Japanese military troops invaded Manchuria and slaughtered over 200,000 innocent civilians in an area where they could obtain many of the resources they needed. This became known as the Nanking Massacre This incensed the US and many nations. The US and other nations place an oil embargo and banks would not loan money to the Japanese if they did not pull out of Manchuria and other places they had invaded.
By December of 1941 the Japanese leaders made the decision to threaten to destroy the US Naval Fleet to prevent the US Navy from coming after them when Japan went after the Dutch East Indies, Singapore, Malaya, Australia and a host of other locations. But when Emperor Shōwa went to D.C, he had a plan. He told his air-force to wait until a certain time to attack or until he said not to. The U.S government told him to wait which put him off schedule and as time went by the Japanese thought that he was kidnapped or worse, dead. So they bombed the harbor.
This was the primary motivation for the Japanese Navy to attack on December 6, 1941.