Can the US and Russia do a deal in the next few months that resolves the core security concerns that trouble both nations? At the end of a busy week in east-west diplomacy, this is the fundamental question that Washington and Moscow face. Put simply, a deal would go something like this. The US would tell Russia that it is halting plans to site a missile shield in Europe – a programme that Moscow has long regarded as a threat, and about which President Barack Obama has justifiable misgivings. In return, Russia would abandon its long-standing ambivalence over Iran’s nuclear programme and tell the Iranians they must immediately suspend it. This is a crude summary, perhaps. But all the signs are that the Obama administration is trying to pull off a deal on these terms.

America’s intentions are easy to understand. Iran is pressing ahead with uranium enrichment and now has enough material to build one nuclear weapon. If enrichment continues into 2010, Israel may attack Iran’s facility at Natanz, an event that would be a calamity for the world. The US and its European allies have urged Iran to suspend the programme without success. However, if Russia can be persuaded to wield a stick – demonstrating to the Iranians that the world is united in opposing their flirtation with nuclear weapons – then a crisis may be averted.