Ping-pong has a special place in Asian diplomacy. Thirty-seven years ago, a US table tennis team visited Beijing and paved the way to the historic Sino-American rapprochement. This month, Hu Jintao went to Tokyo and with a few quickfire rallies against a Japanese champion the Chinese President showed that he can safely boast the most impressive forehand at table tennis of any head of state.

Three week later there has been an extraordinary breakthrough in Sino-Japanese relations unthinkable two months ago: an invitation from Beijing to Japanese soldiers to set foot on Chinese soil for the first time since the Second World War. The Chinese President's request for their help in airlifting tents and relief supplies to the region devastated by the earthquake breaks one of the oldest and most emotional taboos in postwar Asian politics. It has, however, taken the catastrophe of the earthquake to translate the subsequent goodwill into a momentous diplomatic initiative. China turned first to Japan for emergency aid and has welcomed medical teams sent swiftly to the disaster area. But as the scale of the tragedy becomes more apparent and the threat grows of further deaths from aftershocks and flooding from the rupture of dams caused by rockfalls, the Chinese leadership is swiftly casting aside old habits of secrecy and insularity. The unprecedented access granted to foreign relief teams, the television reports from the area and the visits of China's leaders have changed the national mood.

Nowhere has this sudden shift of political tectonics shaken assumptions as much as in Japan. The Chinese Communist Party had long used the bogey of a hostile Japan as a way of rallying public opinion, exploiting wartime memories and missing no opportunity to lambast Japan for its supposed lack of contrition and resurgent nationalism. A nadir was reached in 2005, when, despite extensive trade links, China's newly elected leadership encouraged mass demonstrations against the visits by Junichiro Koizumi, then the Japanese Prime Minister, to the Yasukuni shrine honouring the country's war dead, including 14 executed war criminals. The demonstrations, which threatened to get out of hand, had repercussions across Asia, where Japan's war record is still a sensitive political factor.

The wise decision by Mr Koizumi's two successors not to visit the shrine and a more pragmatic mood in China have, since his departure, prompted a rapid warming in relations. But there are still lingering taboos: the refusal of Japan to acknowledge the extent of its wartime atrocities (especially the Rape of Nanking), the hostility throughout Asia to a more proactive role by Japan's Self-Defence Forces and the stubborn insistence by all sides on sticking to formulaic statements on the Second World War have made real political co-operation difficult. Both China and Japan are governed by a wary mutual suspicion.

Natural disasters, however, can overwhelm political rigidities. Years of antipathy between Greeks and Turks were swept aside by two earthquakes, one in each country, that prompted an outpouring of sympathy and help and changed perceptions. So too the Chinese disaster may change relations with Japan. This is a moment of great Asian humanity. Mr Hu has shown courageous leadership. Japan should now reciprocate.