Who decided to attacked pearl habor?

Answer 1

Japanese Admiral Isoroku Yamamoto. Yamamoto was not the head of the Japanese Navy - he commanded the Combined Fleet. He had to convince his superiors in the Navy and the Navy Ministry to go along with his plan. Yamamoto believed that this was the only chance the Japanese had for victory, if war came with the US. The Japanese planned to conquer southeast Asia and believed this would cause the US to go to war with them. Therefor, Yamamoto believed the US must be attacked as the very first step of the process, to ensure that the US Navy could not interfere with their plans for conquest.

Yamamoto calculated that a serious blow to the US Navy would buy six months time, by the end of which Japan should have consolidated its conquests. This turned out to be just about completely accurate. Yamamoto's superiors and colleagues in the Japanese Navy were divided over the advisability of the attack on Pearl Harbor. It would take most of the striking power of the Japanese Navy far into enemy waters and make these ships unavailable for the main operation, which was conquest of southeast Asia. The Japanese expected that they might lose half the ships in the attack on Hawaii, but Yamamoto believed it was worth the price and had to be done.

This entire Japanese line of thought was based on the questionable assumption that their invasions in southeast Asia would cause the US to go to war with Japan. This is far from certain, that any number of Japanese invasions in Asia would cause the US to abandon isolationism and the people of the US to demand war with Japan. The militarist leaders of Japan could take their country to war whenever they chose, and perhaps they thought that President Roosevelt could do the same. So the Japanese chose the one thing they could do which was guaranteed to make certain that the US did, in fact, go to war with Japan - a direct attack on the US. The US almost certainly would have become involved in WWII somehow, eventually. But the choice made by the Japanese insured immediate American belligerency.