Japan sought to secure the resources in Southeast Asia and much of China and to establish the “Greater East Asia Co-Prosperity Sphere.” Before it accomplish this, Japan believed it necessary to destroy or neutralize American striking power in the Pacific before moving southward and eastward to seize Southeast Asian countries.
Attacking Pearl Harbor was a preemptive strike, performed in order to paralyze the US so it would not be able to interfere in Japanese military operations in southeast Asia. Once in control of these areas, the Japanese intended to establish a defensive perimeter in the Pacific.
Japan thought that the Allies would wear themselves out in frontal assaults against the perimeter and ultimately agree to negotiated peace, leaving Japan in possession of most of its conquests. Of course, in the end, this idea did not work in the favor of the Japanese.
Japanese Military in WWII 