In less than nine months, the 1991 Start I treaty expires. It contains the basic rules of verification that give both Moscow and Washington the confidence that they know the size and location of the other’s nuclear forces.

The Bush administration made little effort to work out a replacement deal. So we are encouraged that American and Russian officials seem to want a new agreement. Given the many strains in the relationship, it will take a strong commitment from both sides, and persistent diplomacy, to get one in time.

When President Obama meets Russia’s president, Dmitri Medvedev, in London on April 1, the two should commit to begin talks immediately and give their negotiators a deadline for finishing up before Dec. 5. For that to happen, the Senate must quickly confirm Mr. Obama’s negotiator, Rose Gottemoeller, so she can start work.

Mr. Bush and then-President Vladimir Putin signed only one arms-control agreement in eight years. It allowed both sides to keep between 1,700 and 2,200 deployed warheads. Further cuts — 1,000 each makes sense for the next phase — would send a clear message to Iran, North Korea and other wannabes that the world’s two main nuclear powers are placing less value on nuclear weapons.

Mr. Obama and Mr. Medvedev should also pledge that these negotiations are just a down payment on a more ambitious effort to reduce their arsenals and rid the world of nuclear weapons. The next round should aim to bring Britain, France and China into the discussions. In time, they will have to cajole and wrestle India, Pakistan and Israel to the table as well.

There is a lot President Obama can do right now to create momentum for serious change. We hope his expected speech on nuclear weapons next month is bold.

He can start by unilaterally taking all of this country’s nuclear weapons off of hair-trigger alert. He should also commit to eliminating the 200 to 300 short-range nuclear weapons this country still has deployed in Europe. That would make it much easier to challenge Russia to reduce its stockpile of at least 3,000 short-range weapons. These arms are unregulated by any treaty and are far too vulnerable to theft.

Mr. Obama must also declare his commitment to include all nuclear weapons in negotiated reductions — including thousands of warheads that are now held in reserve and excluded from cuts. And he must make good on promises to press the Senate to ratify the Comprehensive Test Ban Treaty (opponents are already quietly organizing) and the international community to adopt a pact ending production of weapons-grade nuclear fuel.

Mr. Obama must reaffirm his campaign pledge to transform American nuclear policy that is still mired in cold war thinking. His administration’s nuclear review is due by year’s end. It must make clear that this country has nuclear weapons solely to deter a nuclear attack — and that this administration’s goal is to keep as few as possible as safely as possible. The review must also state clearly that the country has no need for a new nuclear weapon and will not build any.

Two decades after the fall of the Berlin Wall, Russia and the United States together still have more than 20,000 nuclear weapons. It is time to focus on the 21st-century threats: states like Iran building nuclear weapons and terrorists plotting to acquire their own. Until this country convincingly redraws its own nuclear strategy and reduces its arsenal, it will not have the credibility and political weight to confront those threats.

The New York Times