When U.S. President Bush and Russian President Putin signed the Moscow Treaty in 2002, they addressed the nuclear threat by reducing offensive weapons, as their predecessors had. But the Moscow Treaty was different. It came in the wake of America's 2001 withdrawal from the Anti-Ballistic Missile Treaty of 1972, and for the first time the United States and Russia reduced their offensive nuclear weapons with no agreement in place that constrained missile defenses.

Breaking the link between offensive force reductions and limits on defense marked a key moment in the establishment of a new nuclear agenda no longer focused on the Cold War face-off between the Warsaw Pact and NATO. The real threat was that the world's most dangerous weapons could end up in the hands of the world's most dangerous regimes—or of terrorists who would launch attacks more devastating than 9/11. And since those very rogue states also pursued ballistic missiles, defenses would (alongside offensive weapons) be integral to the security of the United States and our allies.

It is in this context that we should consider the potential contribution of the New Start treaty to U.S. national security. The treaty is modest, reducing offensive nuclear weapons to 1,550 on each side—more than enough for deterrence. While the treaty puts limits on launchers, U.S. military commanders have testified that we will be able to maintain a triad of bombers, submarine-based delivery vehicles and land-based delivery vehicles. Moreover, the treaty helpfully reinstates on-site verification of Russian nuclear forces, which lapsed with the expiration of the original Start treaty last year. Meaningful verification was a significant achievement of Presidents Reagan and George H.W. Bush, and its reinstatement is crucial.

Still, there are legitimate concerns about New Start that must and can be addressed in the ratification process and, if the treaty is ratified, in future monitoring of the Obama administration's commitments.

First, smaller forces make the modernization of our nuclear infrastructure even more urgent. Sen. Jon Kyl of Arizona has led a valiant effort in this regard. Thanks to his efforts, roughly $84 billion is being allocated to the Department of Energy's nuclear weapons complex. Ratifying the treaty will help cement these commitments, and Congress should fully fund the president's program. Congress should also support the Defense Department in modernizing our launchers as suggested in the recent defense strategy study coauthored by former Secretary of Defense Bill Perry and former National Security Adviser Stephen Hadley.

Second, the Senate must make absolutely clear that in ratifying this treaty, the U.S. is not re-establishing the Cold War link between offensive forces and missile defenses. New Start's preamble is worrying in this regard, as it recognizes the "interrelationship" of the two. Administration officials have testified that there is no link, and that the treaty will not limit U.S. missile defenses. But Congress should ensure that future Defense Department budgets reflect this.

Moscow contends that only current U.S. missile-defense plans are acceptable under the treaty. But the U.S. must remain fully free to explore and then deploy the best defenses—not just those imagined today. That includes pursuing both potential qualitative breakthroughs and quantitative increases.

I have personally witnessed Moscow's tendency to interpret every utterance as a binding commitment. The Russians need to understand that the U.S. will use the full-range of American technology and talent to improve our ability to intercept and destroy the ballistic missiles of hostile countries.

Russia should be reassured by the fact that its nuclear arsenal is far too sophisticated and large to be degraded by our missile defenses. In addition, the welcome agreements on missile-defense cooperation reached in Lisbon recently between NATO and Russia can improve transparency and allow Moscow and Washington to work together in this field. After all, a North Korean or Iranian missile is not a threat only to the United States, but to international stability broadly.

Ratification of the treaty also should not be sold as a way to buy Moscow's cooperation on other issues. The men in the Kremlin know that loose nukes in the hands of terrorists—some who operate in Russia's unstable south—are dangerous. That alone should give our governments a reason to work together beyond New Start and address the threat from tactical nuclear weapons, which are smaller and more dispersed, and therefore harder to monitor and control. Russia knows too that a nuclear Iran in the volatile Middle East or the further development of North Korea's arsenal is not in its interest. Russia lives in those neighborhoods. That helps explain Moscow's toughening stance toward Tehran and its longstanding concern about Pyongyang.

The issue before the Senate is the place of New Start in America's future security. Nuclear weapons will be with us for a long time. After this treaty, our focus must be on stopping dangerous proliferators—not on further reductions in the U.S. and Russian strategic arsenals, which are really no threat to each other or to international stability.

A modern but smaller nuclear arsenal and increasingly sophisticated defenses are the right bases for U.S. nuclear security (and that of our allies) going forward. With the right commitments and understandings, ratification of the New Start treaty can contribute to this goal. If the Senate enters those commitments and understandings into the record of ratification, New Start deserves bipartisan support, whether in the lame-duck session or next year.

Ms. Rice, a professor of political economy at the Stanford Graduate School of Business and a senior fellow at Stanford's Hoover Institution, was U.S. national security adviser from 2001 to 2005 and U.S. secretary of state from 2005 to 2009.

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