What stopped the civil war between Nationalists and Communists in 1937?

As I guess that this is a question about China, the answer is the invasion of Japan on China.

Explanation:

While China had its internal problems Japan decided to use the situation and to launched an to invade China in 1937. They already controlled Manchuria, so they started bombing China. Many people died from bombing, and later from starvation. In just a year Japan managed to control a large portion of China.