Both Japan and China are being very nationalistic and this is dangerous, but in a way, America has caused it. America asked Japan to officially say their opinion about Taiwan. Japan announced that if China attacks Taiwan and then the U.S. uses its military to protect Taiwan, then Japan will help the U.S.  This means that Japan promises to protect Taiwan even if that means fighting China. China doesn't like this.
  China also doesn't like the fact that Japan may become a member of the U.N. security council. Korea, as well, doesn't like this because of memories from the war, the refusal of some politicians to apologize about the war, and of course the textbook problem.
  On a different point, the U.S. is nervous about China and Japan becoming better friends. Recently, Japan is trading more with China than with the U.S.. In the future, Japan and China could be more powerfull than the U.S. The U.S. wants Japan to continue being friends with the U.S.
  Actually, about Taiwan, Japan has always been on the side of the U.S., but it was never said officially. The U.S. asked Japan to say it officially and Japan did so. Because of this, the textbook problem became 'the straw that broke the camel's back' and has caused people in China to have demonstrations in front of the embassies. It's a very dangerous situation.