How did the Japanese take advantage of fighting between Nationalists and Communists?

The Japanese modernized their military, both with weapons and the way of military training, and that made them by far the most superior military power in the region, or rather the whole continent.

China on the other hand was involved in internal conflict between the Nationalists and Communists, and that has weakened the country significantly. The military was not united, and also still used old weapons and way of training.

The Japanese decided to use the backwardness of their neighbors, as well as the instability in the country, so they attacked Manchuria, the northeastern part of China that has vast fertile plains. Because they were far superior and well organized, while the Chinese were the total opposite of that, they managed to conquer the region with relative ease.