Japanese Expansion :)

Japan was aggressive in their military tactics and were committed to invading China. Even after they won Manchuria back they persisted that they still had a right to invade China. Was it more due to their need for expansion or was it revenge against China for turning to the League of Nations for support in regaining Manchuria? Going back to the failure of the League of Nations, was part of its failure the fact that nations could just withdraw at anytime? Japan easily left and then began attacking China with no issues. Also, why was China's preparation for the attacks of Japan so weak? They had the population and resources to attack, yet for so long the Japanese dominated.