Pacific War
From as early as 1935 Japanese military strategists had concluded the Dutch East Indies were, because of their oil reserves, of considerable importance to Japan. By 1940 they had expanded this to include Indo-China, Malaya, and the Philippines within their concept of the Greater East Asia Co-Prosperity Sphere. Japanese troop build ups in Hainan, Taiwan, and Haiphong were noted, Japanese Army officers were openly talking about an inevitable war, and Admiral Sankichi Takahashi was reported as saying a showdown with the United States was necessary.

Q: What was the importance of the Dutch East Indies to Japan?
oil reserves