Why did japan feel it necessary to strike the us before declaring war?

Answer 1

Background

The attack on Pearl Harbor was intended to neutralize the U.S. Pacific Fleet, and hence protect Japan's advance into Malaya and the Dutch East Indies, where it sought access to natural resources such as oiland rubber.War between Japan and the United States had been a possibility of which each nation had been aware (and developed contingency plans for) since the 1920s, though tensions did not begin to grow seriously until Japan's 1931 invasion of Manchuria. Over the next decade, Japan continued to expand into China, leading to all-out war between those countries in 1937. Japan spent considerable effort trying to isolate China and achieve sufficient resource independence to attain victory on the mainland; the "Southern Operation" was designed to assist these efforts.

Objectives
The 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. Second, it was hoped to buy time for Japan to consolidate its position and increase its naval strength before shipbuilding authorized by the 1940 Vinson-Walsh Act erased any chance of victory.Finally, it was meant to deliver a severe blow to American morale, one which would discourage Americans from committing to a war extending into the western Pacific Ocean and Dutch East Indies. To maximize the effect on morale, battleships were chosen as the main targets, since they were the prestige ships of any navy at the time. The overall intention was to enable Japan to conquer Southeast Asia without interference.

Striking the Pacific Fleet at anchor in Pearl Harbor carried two distinct disadvantages: the targeted ships would be in very shallow water, so it would be relatively easy to salvage and possibly repair them; and most of the crews would survive the attack, since many would be on shore leave or would be rescued from the harbor. A further important disadvantage—this of timing, and known to the Japanese—was the absence from Pearl Harbor of all three of the U.S. Pacific Fleet's aircraft carriers (Enterprise, Lexington, and Saratoga). Ironically, the IJN top command was so imbued with Admiral Mahan's "decisive battle" doctrine—especially that of destroying the maximum number of battleships—that, despite these concerns, Yamamoto decided to press ahead.

Japanese confidence in their ability to achieve a short, victorious war also meant other targets in the harbor, especially the navy yard, oil tank farms, and submarine base, could safely be ignored, since—by their thinking—the war would be over before the influence of these facilities would be felt.

Basically, the Japanese Empire was looking to conquer all of Asia and saw that the United States government was in the way.