What price for a nuclear-free world?

In his visit to Moscow this week, President Obama laid down the first marker in his quest to free the world of all nuclear weapons – which may be the defining goal of his foreign policy.

Russia and the US agreed Monday to replace a 1991 nuclear-arms treaty that expires Dec. 5 with one that would further reduce the number of each nation's atomic warheads as well as the missiles, planes, and submarines that can deliver them.

The possible cuts would be more of a moral symbol for other nations to follow than a real cutback that would make the world safer. Even if successful, a new Strategic Arms Reduction Treaty (or START II) between Russia and the US would leave their combined number of warheads at levels far above those of other nuclear nations and large enough to obliterate either nation many times over.

But the more immediate point is that the exact size of the cuts is still to be negotiated. That leaves plenty of diplomatic wiggle room in the months ahead for the Kremlin to take advantage of Mr. Obama's zeal for a nuclear-free world. It can extract concessions that would help Russia become a great power again – especially a power over its neighbors.

Obama must be careful what he wishes for. He should not give up other vital American interests as he tries to bring about nuclear disarmament. He seemed to have already lost, for example, the battle over whether to link the START II negotiations to Russia's concern about America's missile defense plans in Europe for fending off Iranian rockets.

And he agreed to a preliminary goal in warhead cuts that could allow Russia to reduce the number by only 25. START I limited the number of warheads to between 1,700 and 2,200, and the proposed upper limit now is 1,675.

Meanwhile, in delivery systems – where the US is much stronger – the proposed cuts are large: from the current range of 1,500 to 1,675 units to between 500 and 1,100 units.

Such American concessions to Moscow reveal an Obama administration eager for almost any bilateral nuclear-arms treaty that can create momentum toward global nuclear disarmament.

And Obama's Pentagon has not yet completed the Nuclear Posture Review that would help Congress decide what sort of strategic forces the US needs for the next few years. Will China, for instance, decide to greatly expand its small nuclear arsenal if it can approach the new, lower number of Russian or US warheads achieved with a START II treaty?

Obama is wise to engage the Kremlin fully on a new START treaty. The mutual inspections of each country's arsenals under the current pact cannot be allowed to discontinue and, more important, the US can at least learn what linkages and concessions are sought by Moscow.

But the Russians saw Obama and his antinuke idealism coming. As with the battle against climate change, he must use persuasion in creating a better world rather than negotiating with giveaways that may, in the end, not achieve his grand vision.