If President Obama really manages to pull together a deal with Russia that cuts each country’s stockpile of nuclear warheads by four fifths it will be an astonishing feat, and one that will give him a claim to have set US foreign policy on a very different course.

The time is right. The present arms deal runs out at the end of this year. Both countries have far more warheads than they need for deterrence and could do with saving money (this is an understatement) from that part of the defence budget. It would also put the US in a far stronger position to argue against nuclear proliferation — in Iran, in North Korea, and across the Arab world, where leaders have shown quiet interest.

All the same, it is an imaginative move by Obama — another word for surprising — and it will put Vladimir Putin, the Russian Prime Minister, on the back foot. Putin has clearly not decided whether to welcome the new team in the White House or to continue with his recent sour and obstructive approach to requests by the US and European Union.

Obama has one prize to dangle — not installing the missile defence system in Eastern Europe that has so annoyed Moscow. He probably wants to do this anyway, on cost grounds, and because many are sceptical that it would be any use in protecting the US or Europe against the supposed threat from Iranian missiles. But he needs to extract as good a price as he can get.

Analysts say he also wants to ask Russia for help on Iran. Obama, short of ways to persuade Tehran to drop its nuclear work, which the West believes is a covert weapons programme, wants Russia to stop supplying parts for Iran’s reactor or missile components. A reduction in nuclear weapons, and a cut in help to Iran, in return for scrapping the shield: that’s the outline of the offer. The question is whether Putin will think that he is being asked for too much in return for something that Obama probably wants to do anyway.

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