Mainly for resources and racial views .
A. During the WW2 era, Japan needed resources very badly...it was a small little island nation that had little resources, and a smaller population for it's rival nations. Japan needed raw materials for its industries, and it claimed that it wanted to guide other Asian countries to obtain prosperity
B. For the racial reasons, the Japanese at the time thought, (much like the Germans in Nazi Germany, but Japan's views are the lesser known ones in the Western world) that the Japanese were the supreme Asian race. Japan looked at others and planned to either "eradicate or "lead"" the other Asian, and other nations of the world. These racist views led to nightmarish like killings and invasions of Chinese, Burmese, and people from the Pacific Islands (aka Pacific Islanders). At this time, sadly with the advantages, the Japanese raped and sexually assaulted many women and put them in sex slavery, which goes under human trafficking. Things are also some of the reasons the Japanese government keeps apologizing for WW2.
