Agreement by Barack Obama and Dmitry Medvedev, the US and Russian presidents, on a fast-track negotiation to substantially reduce their arsenals of strategic nuclear weapons is good news. It is a realistic way of pressing the “reset” button in relations between the old cold war adversaries to end the increasingly bitter recriminations of recent years. But even if they manage to do a deal by the end of the year, it will leave many tough and divisive questions unresolved.

The present strategic arms reduction treaty (Start) expires in December. It was negotiated and signed in the dying days of the Soviet Union. As the two leaders said after their meeting in London on Wednesday, before the G20 summit, it has long fulfilled its intended purpose. Both sides have reduced their long-range nuclear missiles far below the ceilings they agreed then. Russia has some 2,800 still deployed and the US around 2,400. They plan to cut those ceilings by about one third. They can both afford to be even more ambitious.