why did japan enter world war II?

Japan was not a nation blessed with many natural resources as was the U.S., and other enemies of that nation. In order to secure those additional raw materials, such as rubber, tin, and petroleum, among others, the war leaders decided that conquest of other nations was a solution, and began by attacking China, which in actuality was the (early) start of the war.

At the time, China was a virtual colony for many European nations, including England, France, Netherlands, and the US as well as Japan. Other colonies in Asia included Vietnam for France, Philippines for US, Indonesia for Netherlands, and so on. As the great powers competed for regional interests, Japan was quickly gaining ground from obtaining German concessions after participating on the allied side in WWI. The United States did not view Japan's intrusion into China as favorable to its own interests in the area, and thus economic frictions between US and Japan arose. At the time, Japan relied 80% of its resources, including oil, indispensable at the time, to the US. The US began throttling its exports to Japan, and pressured its allies to do the same (such as the Netherlands and Mexico, which Japan sought to purchase oil as alternative sources). The friction reached its height when the US stopped all exports to Japan under the Export Control Act on July 31, 1940 after freezing all Japanese assets in the US, virtually cutting off all of Japan's access to resources. 4 months later, Japan decides to attack the United States on Pearl Harbor.