It is tempting to dismiss the latest arms control treaty between the United States and Russia as more flash than bang. After all, it leaves thousands of weapons in each country’s arsenal, eliminates weapons that both governments would have likely cut anyway and there is no guarantee that either legislature will agree to the deal.

Resist that temptation. The treaty reduces deployed nuclear arsenals by 30 percent. More important, it provides a much-needed boost to arms control efforts and injects momentum into global negotiations that will be under way this spring. As U.S. President Barack Obama noted at the conclusion of the negotiations, the deal is “the most comprehensive arms control agreement in nearly two decades.”

Mr. Obama took office determined to reshape his country’s thinking about nuclear weapons. His new approach was revealed in the speech he delivered in Prague in April 2009 in which he outlined his vision of a world free of nuclear weapons. His Russian counterpart, President Dmitry Medvedev took up the challenge and the two men resumed negotiations on reducing strategic nuclear arsenals. Details proved difficult as ever, and a deal that was to have been concluded by year’s end — when the 1991 Strategic Arms Reduction Treaty (START) expired — remained out of reach until a few weeks ago.

A renewed push produced agreement on a treaty that caps the two countries’ deployed warheads at 1,550, a 74-percent cut from the 1991 treaty, and 30 percent below the levels agreed in the Moscow Treaty that was signed in 2002 and expires in 2012 (and does not require either country to actually destroy the warheads shelved). In addition, it limits the number of launchers that either side can deploy to 800, half the currently permitted total.

Critics dismiss the value of the agreement. They note that the deal only codifies unilateral policies already in place, and that the two countries are not necessarily cutting their arsenals. Russia, for example, only has about 800 launchers anyway. Critics complain that the deal does not include thousands of tactical weapons or strategic warheads that are stored. And they worry that conservatives in each country will block ratification.

Republicans in the U.S. challenge any arms control agreement as a threat to national security: They resist any restraint on U.S. freedom of maneuver. Their instincts are reinforced by Russian objections to U.S. missile defense plans, which Moscow fears could neutralize its arsenal’s power as a deterrent. Finally, there is Republicans’ reluctance to give Mr. Obama credit for any foreign policy initiative.

Russian conservatives fear that limits on their strategic arsenals will be greater if the U.S. pursues missile defense. Moreover, they note that their country is increasing its reliance on nuclear weapons, a consequence of the deterioration of the Russian military in the aftermath of the Cold War. The U.S., in contrast, is cutting its reliance on nuclear weapons.

Concerns about the impact of missile defense was the chief obstacle to a deal. U.S. negotiators knew they could not give up options in this regard and still get the agreement through the Senate. And indeed, those negotiators insist that missile defense capabilities remain unaffected. Reportedly, it took direct intervention by the two presidents to overcome the deadlock.

Motivating the two men, in addition to the expiration of the 1991 START agreement, were several looming deadlines. The first is the upcoming nuclear security summit that Mr. Obama will host in mid-April. The second is the Nuclear Nonproliferation Treaty Review Conference, which will be held at the United Nations in early May. Hanging over all those meetings is North Korea’s continued defiance of the world with regard to its nuclear program and Iran’s suspected nuclear ambitions.

The March 26 deal sealed by Mr. Obama and Mr. Medvedev injects a much-needed boost to those upcoming talks. It shows that the possessors of the world’s two largest arsenals — holders of more than 90 percent of the world’s nuclear weapons — are serious about working toward a nuclear-free world. Both men have said that this deal, once ratified, will open the door to broader nuclear arms-reductions talks.

Equally significant, the agreement suggests that the U.S. and Russia have “reset” their relationship, and demonstrates that they can work together in service of their national interests and the common good. That is an important shift after a long and disturbing slide in relations.

Hopefully, the two governments can now coordinate their positions and press Pyongyang and Tehran to hew to international standards and reinforce the global nonproliferation regime.

That effort can only succeed if all countries — those with nuclear weapons and those without — recognize their responsibility to help stop the spread of those weapons. Washington and Moscow have much more to achieve, but they are off to a good start.

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