Imagine that China decided to land soldiers on the disputed islands that it calls the Diaoyus. Japan, which administers the uninhabited rocks and knows them as the Senkakus, might, under its own laws, be unable to meet the incursion with force. The coastguard may repel private vessels, but not troops arriving from the air or from a submarine. It is not clear whether Japan's pacifist constitution prevents its Self-Defence Forces from striking back until its own citizens are injured. Nor is it obvious that its main ally, America, would go to war to rid the Senkakus of the platoon of Chinese troops. This uncertainty is dangerous. Shinzo Abe, the prime minister, should be trying to allay fears. He has chosen instead to visit a shrine commemorating high-ranking war criminals.