Why did Japan decide to attack Pearl Harbor in 1941?

Answer: At Pearl Harbor itself, there were incidents that, properly interpreted, might have given a brief warning. Four hours before the decisive moment, a Japanese submarine was sighted by the minesweeper USS Condor. About two and a half hours later, the commander of the destroyer USS Ward sent a message saying that he "had attacked, fired upon, and dropped depth charges upon submarine operating in defensive sea area" near Pearl Harbor. While Kimmel waited for confirmation of this report, the Japanese opened hostilities. In these same morning hours, U. S. Army Pvt. George Elliott, practicing on the radar set after its normal closing time, noticed a large flight of planes on the screen. When he telephoned his lieutenant, he was told to disregard the observation, as a flight of B-17 bombers from the United States was expected at that time. Once again an opportunity was missed.

Objectives. The Japanese attack had several major aims. First, it intended to destroy important American fleet units, thereby preventing the Pacific Fleet from interfering with Japanese conquest of the Dutch East Indies and Malaya and to enable Japan to conquer Southeast Asia without interference.