Japan's ambitions for their empire required a great deal of raw materials. The Japan home islands couldn't provide those materials but the resource rich islands of South East Asia as well as Indochina could. While Japan didn't need to declare war on the United States to invade and occupy those places, it was unlikely that the United States would sit idly by while Japan went about conquering half of the Pacific. The United States had already taken a stand against Japanese aggression in the region by seizing Japanese assets in the United States in response top the Japanese occupation of French Indochina as well as put an embargo on the sale of certain commodities to Japan. This seriously jeopardised Japan's ability to wage war as it restricted Japanese access to oil imports.

These actions on the part of the United States convinced the Japanese that further moves into South East Asia would liekly provoke a military response from the US whose possession of the Phillipines as well as various islands throughout the Pacific posed a threat to Japanese lines of communication. The attack on Pearl Harbor was intended to preempt any US military intervention in the region and destroy the US ability to wage war in the region until Japan had completed its conquest and was able to devote its full resources to resisting American military power. The key objective of the Pearl Harbor attack was the destruction of the US fleet carriers, the USS Enterprise, Lexington and Saratoga. While the destruction of the US battleships at anchor in the harbor was also an objective, the Japanese recognised perhaps better then any other power in the war that naval warfare had shifted strongly in favour of aircraft carriers rather then battleships so the destruction of US naval aviation power was the higher priority.

It was well recognised that the Japanese could never hope to contend with US industrial capacity so striking a sudden and critical blow against the US was seen as the only way to win a potential conflict with the nation. It was also hoped that such a blow would bring the United States to the negotiating table and perhaps see the US give Japan a free hand in South East Asia. This thought was based on a hope that such a heavy blow would impact American morale in such a way that the US government was forced to find a peaceful response.

Source: The Second World War by Antony Beevor