The plight of the Kursk highlights the dangers of Russia's ageing weaponry

The plight of the Russian nuclear submarine Kursk and the race against time to save the 116 crew trapped on the seabed is not just a terrible human drama. It is a metaphor for the decline of a superpower, and for the decay of Russia's armed forces since the collapse of the former Soviet Union. And nowhere is that decay more perilous than in the upkeep of Russia's nuclear deterrent.

The plight of the Russian nuclear submarine Kursk and the race against time to save the 116 crew trapped on the seabed is not just a terrible human drama. It is a metaphor for the decline of a superpower, and for the decay of Russia's armed forces since the collapse of the former Soviet Union. And nowhere is that decay more perilous than in the upkeep of Russia's nuclear deterrent.

Despite the shrinkage of its military since 1991, Russia still has a million men under arms and a nuclear arsenal of up to 6,000 warheads, all of which must be maintained out of a declared defence budget of around £3bn at current exchange rates. Assuming that the Soviet penchant for secrecy has not changed, we may suppose that the real expenditure is three or four times that much. Even so, it does not remotely compare to the Pentagon's $190bn budget, or even Britain's £22bn annual spending on defence.

And the story gets worse. Pay is erratic; budgeted funds do not materialise; hazing and corruption are rife in the ranks. Moscow has done its best to protect its strategic nuclear forces from such vagaries but cannot remotely afford to carry out the costly decommissioning of warheads, missiles and submarines required by arms-control agreements. The disaster on the Kursk, apparently caused by an on-board explosion, proves that even the most modern parts of the Russian military machine are vulnerable. It also reinforces the case for further sweeping reductions in nuclear weapons, which President Putin has grasped quickly.

It is no coincidence that, last Friday, just two days before the accident, Mr Putin appears to have bowed to the inevitable. If reports are correct, a potentially historic decision has been taken. Russia's bloated, increasingly obsolescent nuclear deterrent will be scaled back and resources diverted to strengthen its conventional forces, whose inadequacies have been humiliatingly exposed in two Chechen wars. Moscow is now prepared to let its total warhead stock fall to 1,500 or even fewer, a fraction of the 3,500 it is permitted under the Start-II treaty.

Obviously, the danger exists that a Russia with better-equipped and more mobile conventional forces will be more inclined to cause trouble. But that remaking of its ramshackle military will take a decade at least. Any risk in the meantime is far outweighed by the advantages.

Properly handled, the plight of the Kursk could thus give momentum to the cause of large-scale arms reductions. Nuclear weapons are easy to eliminate on paper. The expensive and protracted part is their physical dismantlement and the making-safe of reactors and weapons-grade fissile material. America is putting together a $2bn package to help to dispose of 34 tons of Russian weapons-grade plutonium. More is needed.

By accepting American or British help in rescuing its stricken submarine, Russia could demonstrate that Cold War bygones are truly bygones. In return, as economic reality forces Moscow to slash its deterrent, the US should begin to do likewise without waiting for a Start-III agreement. If it does, a disaster in the Barents Sea may actually help to make the world a little safer.