Vladimir Putin makes an unlikely swinger. Until now, Russia’s president has been assiduously courting China. He sells Beijing oil and weaponry. Bilateral trade will be worth about £15bn this year. Cold war border disputes have been settled. The two countries recently held unprecedented joint military exercises. And they often act together at the UN on issues such as Iran and Sudan.Yet risking accusations of infidelity from Beijing, Mr Putin has spent the past three days in Japan, China’s old enemy and regional rival and a country with which Russia is technically still at war. He promised to build a pipeline linking Japan to Siberia’s oil. He encouraged further investment in Russia. And he was conciliatory over the hot-button issue of the Kurile islands, known in Japan as the Northern Territories, seized by the Soviet Union in 1945 and which Tokyo wants returned.