Last weekend's speech by President Barack Obama, embracing the goal of eliminating all nuclear weapons, reflects a developing view around the world that the time is ripe to revive what has in the past been considered a utopian dream. We are now a long way from the massive arsenals of the cold war, and the Americans and Russians are about to embark on a new round of talks that could see arsenals reduced to at most 1,500 warheads each and possibly lower. Yet the move towards zero is by no means free of risk. The problem is less the ultimate goal, and whether it could be properly verified, but the transition period, which we have already entered. This new nuclear age involves issues and perils quite different from those of the cold war.

The old nuclear age was dominated by the arsenals of the US and Russia, with tens of thousands of warheads on both sides, and the drive to perfect their lethality while exploring forms of defence and relying on deterrence. Confidence that this relationship could be stable came from mutual assured destruction, with caution resulting from the prospect of terrible retribution even after a surprise first strike. The biggest fear was that numerous nuclear detonations would hurl vast quantities of soot and smoke into the stratosphere, creating a nuclear winter.

The new nuclear age involves more countries but smaller arsenals. The old strategic theories argued that such a situation was bound to be less stable than the cold war. Two superpowers locked in a long-term confrontationunderstood the logic of deterrence. Large arsenals encouraged caution as there could be no doubting the catastrophic consequences should they be fired. With small arsenals there might be a belief that a cleverly aimed strike could destroy the enemy's means of retaliation or that somehow the use of a few weapons, however terrible their effects, could be part of warfare as usual and contained in their impact. As a result of proliferation, there are now nuclear states marked by chronic insecurity, with tumultuous internal politics and fraught external relations.