During WW1, Germany had occupied and established many island colonies and a few colonies in China. On the other hand, Japan wanted the Chinese land Germany had to itself, so the country joined the Allied Powers to fight against Germany and take the land. Throughout WW1, Japan was extremely victorious, helping to defeat many German colonies, and also in getting to keep all of the land they conquered through the Treaty of Versailles. Even the ports in China were taken over by the Japanese empire. These acts greatly irritated China, and sparked a conflict between the two. Japan’s sights were now set on China for expansion - now that the Pacific Ocean was basically theirs.