Relations between China and Japan are rarely better than acidic, and last week brought another reminder of how easily they can turn venomous. In Tokyo about 150 parliamentarians attended the Yasukuni shrine, which pays homage to hundreds of Japanese war criminals as well as the country’s war dead, drawing shrieks of protest from Beijing. Yet the persistent enmity between the two countries is bewildering to many outsiders, rooted in squabbles so obscure that they can even look manufactured.

Foreign statesmen have to tread a fine line. Consider the collection of uninhabited islands in the East China Sea that both countries claim as their own. President Barack Obama stated last week that the barren archipelago fell under a defence treaty that obliges America to come to Japan’s aid in the event of a military attack, the first time that Washington had offered such an assurance. But Mr Obama also warned Tokyo against provoking China. His intervention is unlikely to have pleased either side.

Another prominent American seems to have gone out of his way to irritate both adversaries. Oliver Stone, the Hollywood director whose films include Born on the Fourth of July, last week berated Chinese film-makers for failing to probe their country’s history. Not so long ago it was the Japanese he was upbraiding, for what he said were inadequate apologies for past acts of war.