日本為何偷襲珍珠港？資源/經濟是公認的原因。美國支持中國是一個很重要的原因。日本的目標實際上是和美國達成一個妥協。

關於日本為何偷襲珍珠港會有很多說法。但日本追求資源和美國對日本經濟限製是公認的原因。美國向國民黨提供經濟支持。這必然幫助中國抗戰延遲日本占領中國的計劃。日本的計劃是有限的war。目的是想有機會和美國達成和平（negotiated peace）。

Why Did Japan Attack Pearl Harbor? - History.com
https://www.history.com/news/why-did-japan-attack-pearl-harbor
The U.S. Was Trying to Stop Japan’s Global Expansion
In light of such atrocities, the United States began passing economic sanctions against Japan, including trade embargoes on aircraft exports, oil and scrap metal, among other key goods, and gave economic support to Guomindang forces. 

Tokyo and Washington negotiated for months leading up to the Pearl Harbor attack, without success. While the United States hoped embargoes on oil and other key goods would lead Japan to halt its expansionism, the sanctions and other penalties actually convinced Japan to stand its ground, and stirred up the anger of its people against continued Western interference in Asian affairs.

https://pearlharborwarbirds.com/why-japan-attacked-pearl-harbor/
Here are 3 reasons why Japan attacked Pearl Harbor:
Reason #1: An Increased Need For Natural Resources
Japan had an increased need for natural resources like oil, minerals and steel as their goals for expansion in Asia and the Pacific increased.
Reason #2: Restrictions
The United States also had an obvious interest in these natural resources, and in response to to the Japanese aggression, the U.S. Congress placed restrictions on doing business with Japan. And, if that weren’t enough, Japanese assets in the United States were frozen.
Reason #3: Expansion in the Pacific
President Roosevelt moved the US Pacific Fleet from California to Pearl Harbor in 1939. This move was a threat to Japan, who wanted to expand in the Pacific. Military leaders and politicians saw a war between the U.S. and Japan as inevitable, with the solution being to attack first. Japan did just that。

Pacific war- wiki
https://en.wikipedia.org/wiki/Pacific_War
In preparation for the war against the United States, which would be decided at sea and in the air, Japan increased its naval budget as well as putting large formations of the Army and its attached air force under navy command. While formerly the IJA consumed the lion's share of the state's military budget due to the secondary role of the IJN in Japan's campaign against China (with a 73/27 split in 1940), from 1942 to 1945 there would instead be a roughly 60/40 split in funds between the army and the navy.[71] Japan's key objective during the initial part of the conflict was to seize economic resources in the Dutch East Indies and Malaya which offered Japan a way to escape the effects of the Allied embargo.[72] This was known as the Southern Plan. It was also decided—because of the close relationship between the United Kingdom and United States,[73][74] and the (mistaken[73]) belief that the US would inevitably become involved—that Japan would also require taking the Philippines, Wake and Guam.
Japanese planning was for fighting a limited war where Japan would seize key objectives and then establish a defensive perimeter to defeat Allied counterattacks, which in turn would lead to a negotiated peace.[75] The attack on the US Pacific Fleet at Pearl Harbor, Hawaii, by carrier-based aircraft of the Combined Fleet was intended to give the Japanese time to complete a perimeter.