President George W.Bush, and his aides, could hardly wait to get rid of all those tiresome arms-control treaties when they took office. They tore up the 1972 antiballistic missile treaty to make way for a still largely imaginary missile defense system. They opposed the Comprehensive Nuclear Test Ban Treaty and never made a serious effort to win a ban on the production of fissile material (the core of a nuclear weapon).

Mr.Bush grudgingly signed his one and only arms-reduction treaty with the Russians in 2002. That means that today - 20 years after the fall of the Berlin Wall - the United States and Russia still have more than 20,000 nuclear weapons, with thousands ready to launch within minutes. The bad news, of course, didn’t stop there.

While Mr.Bush and his team were ridiculing treaties and arms control negotiations as“old think,”North Korea tested a nuclear device, Iran has been working overtime to produce nuclear fuel (usable for a reactor or a bomb) and many other countries are weighing whether they need to get into the nuclear game.

President Obama pledged to address these dangers when he was campaigning. In her recent confirmation hearing, Secretary of State Hillary Rodham Clinton argued that this country’s best hope of doing that is to restore treaties and a rules-based system. Now they have to translate that lofty intent into urgent action.

The first challenge is Russia, the only other country besides the United States with enough weapons to blow up the planet. The administration can start by negotiating a follow-on to the 1991 Start Treaty, which is set to expire in December. The pact contains the only rules for verifying any nuclear agreement, and it provides an opportunity for making even deeper cuts.

The two sides could easily go to 1,000 weapons each in this next round, down from the 1,700 to 2,200 deployed weapons agreed on in the 2002 Moscow treaty. Even without any negotiations, the two countries can immediately take their weapons off high alert.

We applaud the administration’s pledge to work for Senate ratification of the Comprehensive Test Ban Treaty and to revive negotiations on a fissile material production ban. Neither will be easy to achieve, but both are essential if Mr.Obama is serious about reining in a frightening new world of ever-expanding nuclear appetites.

During the campaign, Mr.Obama opposed plans to build a new nuclear warhead. He was right. There is no military or scientific need. Defense Secretary Robert Gates is a strong advocate of the program. Mr.Obama should resist. If the United States is going to have any credibility in arguing that others must restrain their nuclear ambitions, it must restrain its own.

Mr.Bush repeatedly warned about the dangers of nuclear weapons falling into the hands of terrorists. He was right. But he never put in place the strategy needed to ensure that that never happens. And he weakened some of this country’s most fundamental defenses, including its credibility.

President Obama must do better. He can start by restoring the rules of the game.