By Paul Reynolds
World affairs correspondent, BBC News website
The United States and Russia have pressed the reset button on their relationship by reaching for the easiest issue they can settle quite quickly - a reduction in their nuclear forces.
The nuclear deal was something both sides could easily achieve
Nuclear warheads are the low-hanging fruit on the tree of their relations and, after their meeting in London, Presidents Obama and Medvedev announced that they intend to reach a new agreement by December.
Why is this the easiest issue? Because it is in the interest of both sides to make further reductions. They do not need so many.
The current aim, reached in an agreement between Presidents George W Bush and Putin in Moscow in 2002 (and known as the Strategic Offensive Reductions Treaty or Sort) is to cut deployed warheads to between 1,700 and 2,200 on each side by 2012.
The new aim is to get an agreement to take those numbers much lower. It will still give them both the power to destroy each other several times over.
And why by December? Because that is when a previous treaty known as Start (Strategic Arms Reduction Treaty) runs out. Start made dramatic reductions in nuclear forces of some 80% but the key point at this moment is that it contains binding agreements on verification and when the treaty runs out so do those commitments. They will have to be renewed if arms control is to be under proper monitoring.
That creates a timetable and an opportunity which both leaders have seized.
Some problems remain to be overcome. Russia wants to go back to the original idea used in Start and reduce delivery systems, as they are called - rockets, submarines etc - and not just warheads.
It also wants all warheads, not just those ready for use, to be counted. That creates verification problems.
For the United States, it will have to be clear that there is no linkage between this potential agreement and the proposed US anti-missile system in Eastern Europe to which Moscow objects. It is still possible that President Obama will cancel or delay the system, but he will not do so at Russia's behest.
"This is an extremely short timeframe," said Mark Fitzpatrick, nuclear watcher at the International Institute for Strategic Studies in London. "In the past these treaties have taken years to negotiate and ratification also takes a long time but there is political will behind this one."
Reduction not elimination
President Obama hopes that a by-product of a new agreement will be the deflection of criticism that the US is not doing enough to meet its commitment to nuclear disarmament under the Nuclear Non Proliferation Treaty.
A nuclear weapons-free world remains pie in the sky because there are - and will be - missiles in the sky instead
The treaty demands that nuclear-armed states aim for disarmament under a general agreement. A review conference on the treaty is to be held next year.
"The treaty needs political strengthening," said Mark Fitzpatrick. "It is under siege. The last review conference in 2005 was a failure and non nuclear-armed states are losing faith that the nuclear states are fulfilling their side of the bargain."
However, the reality is that the nuclear-armed states might disarm partially but will not disarm completely.
They are all in the process of modernising their forces which will reduce them in numbers - but not eliminate them. A nuclear weapons-free world remains pie in the sky because there are - and will be - missiles in the sky instead.