The world took a giant step toward disarmament this week when U.S. President George W. Bush and his Russian counterpart, President Vladimir Putin, agreed to slash their nuclear arsenals. The deep cuts were the most significant outcome of a three-day summit that seems to herald a new era in relations between the two countries. The bold move could not overcome all the differences between the two leaders, but those that remain, such as over the Antiballistic Missile Treaty, can be worked out -- if there is political will. It is up to Mr. Bush and Mr. Putin to prove to the world that they are willing to do just that.

The agreement to cut the two sides' nuclear weapons came on the first day of the meeting. Although the decision will reduce each country's stockpile by two-thirds -- each side will have fewer than 2,200 warheads -- it was not a surprise. Russia cannot afford to maintain its bloated nuclear arsenal and Mr. Putin had signaled that he was prepared to make deep cuts to about 1,500 warheads. Mr. Bush's decision to match him was widely anticipated, too. With Russian cuts forthcoming, there was no need for the U.S. to maintain its huge stockpile of weapons. Moreover, cuts would encourage Mr. Putin to accept Mr. Bush's plans to develop a missile defense system and scuttle the ABM treaty.

Unfortunately for Mr. Bush, Mr. Putin has shown no inclination to be so accommodating, although that might be a negotiating ploy. The Russian president knows that Mr. Bush is going to develop a missile defense program and Mr. Putin cannot stop him. In this situation, it makes more sense to hold out to maximize his leverage. In previous comments, Mr. Putin indicated his willingness to deal, so more concessions might be forthcoming as the talks progress.