Causes

1. The Japanese made some very aggressive actions against the Chinese. Chief among these was the "Rape of Nanjing" in which they killed, massacred, and raped citizens of the Chinese City of Nanjing.
2. The Japanese burned their own railroad, and blamed it on the Chinese in order to instigate conflict.
3. A rising philosophy in Japan was the hakko ichiu, which promoting bringing all of Asia under the control of the Imperial Emperor. Such a feat was obviously not possible without some form of war.
4. After WWI, through the Great Depression and onwards, Japan’s government became increasingly militaristic. Such a government encouraged conquering other nations in order to reinforce japan’s military might and thus, maintain their own control over the state.
5. Japan’s developing economy was heavily dependant on foreign countries for such things as arms. In order to remove this “weakening’ dependence on countries such as the US, Japan sought to invade other resource-rich countries and create prosperous colonies that would make Japan strong.