Why did Japan invade China before World War II?
In the early part of the 20th Century, Japan's leaders aspired to build a world empire. Empires require conquest of new lands, and symbolically, China was a strong candidate. China was the traditional realm of power in the east for time immemorial. To conquer China would be a huge figurative victory for the Japanese.
Aside from metaphorical nature of a victory over China, the invasion had a military and economic purpose. The region of China acquired by Japan was rich in natural resources, particularly oil. Resources are required to fuel industrial growth and to supply the military. Japan's lack of natural resources on its own mainland was problematic to the goals of imperial growth. For this reason, Japan looked toward China to solve this problem.
After defeating China, Japan installed a puppet government in Manchuria. Aside from the beneficial flow of resources and revenue from China, negative consequences existed. The incursion into China greatly angered the United States and the European industrial powers whose economic interests were tied to the Far East. The United States placed embargos on Japan that caused greater animus between the two countries. This would ultimately lead to American entry into World War II.