The mainland's security forces have many skills, not least curbing dissent. And yet violent anti-Japanese protests seem to be able to erupt spontaneously and regularly in recent days. It is hard to escape the conclusion that if there is no official involvement in their creation, they are at least being sanctioned. There is no doubt that Chinese feelings towards Japan run deep, but these protests involve Japanese citizens on the mainland being threatened and their businesses and property vandalised, all over disputed islands in the East China Sea. Whatever the merits of the dispute, physically endangering foreigners - guests in the country - is no way to move the issue forward.

This is an approach that has been seen before - for example five years ago when Japanese school textbooks were published glossing over wartime atrocities against China and Korea. Demonstrations have occasionally taken place whenever Japanese politicians visit the Yasukuni shrine honouring Japan's war dead. Banner-carrying and slogan-chanting crowds of thousands of mostly young people form in the blink of an eye, march on Japanese companies or diplomatic buildings and, when anger rises, throw stones and cause damage. That is what happened at the weekend in Chengdu , in nearby Mianyang and at Xian. Protests have since erupted in Wuhan and other second-tier cities.

Sovereignty is an emotive issue. It is especially so when Japan is involved; its invasion and occupation of China before and during the second world war remain a matter of the deepest animosity. The brutality of those years, for which Beijing says an appropriate apology has still not been given, remains at the core of disputes. It is not surprising that Japan's arrest and release amid a diplomatic storm of the captain of a trawler involved in a collision near the Japan-claimed Diaoyu Islands has ruptured relations.