When Secretary of Defense Chuck Hagel announced this month that the Pentagon would increase the number of missile interceptors in Alaska, he noted that the U.S. missile defense program in Europe would be restructured. This means cancellation of Phase 4 of the plan, which called for the deployment of upgraded interceptors in Eastern Europe.

The decision could open the way for resolving U.S.-Russian differences over missile defense, one of the thorniest problems on the bilateral agenda, and remove an obstacle to further nuclear arms reductions — if Moscow can say something other than “nyet.”

The initial Russian reaction gave little ground for optimism. But Russian officials often react slowly to new ideas, so we may not yet have the final word.

The Obama administration unveiled its “European Phased Adaptive Approach” in 2009 with the goal of deploying increasingly capable SM-3 missile interceptors in anticipation that Iran would develop missiles with increasingly longer ranges. Moscow initially appeared to welcome the approach.

In November 2010, NATO and Russia agreed to explore a cooperative missile defense for Europe. Talks between U.S. and Russian officials in early 2011 yielded significant convergence on questions such as transparency, joint exercises and jointly manned NATO-Russia centers to share early warning data and plan how NATO and Russia missile defense systems would work together.

The dialogue stalled, however, as Russian officials began to complain more vociferously about Phase 4 of the plan, originally scheduled for 2020, when the SM-3 IIB interceptor would achieve the capability to engage intercontinental ballistic missiles.

Moscow asserted that Iran stood many years, if not decades, from developing an ICBM, and claimed that the United States instead planned to target SM-3 IIBs against Russian ICBMs. U.S. officials countered that SM-3 IIBs in Europe would be ill-placed to engage Russian strategic missiles.

Hagel’s announcement renders that argument moot.

Now if the Russians do not want to move forward on resolving their differences with Washington over missile defense, they have to find other reasons to object. And they may.

A Russian official has expressed opposition to the fact that Phases 2 and 3 of the missile defense plan will go forward in Romania and Poland. SM-3 interceptors in those phases, however, will only be able to engage intermediate-range missiles. That presumably poses no problem for Moscow, as a 1987 treaty bans Russia (and the United States) from having intermediate-range missiles.

Russian recalcitrance may reflect simmering resentment about NATO enlargement, and the prospective deployment of SM-3 missile interceptors in Eastern Europe could add to the unhappiness. But how will small U.S. military detachments with interceptors to defend against missiles that Russia does not have pose a threat to Russia?

Moscow has sought a “legal guarantee” that U.S. missile defenses would not be directed against Russian strategic missiles, even though they know full well that Senate Republicans would block such a treaty. Russian officials assert that the absence of legally binding limits creates uncertainty about the offense-defense relationship.

Moscow is correct that increasing missile defense capabilities could undermine the balance in strategic offensive forces, but that problem will not arise for 15 or 20 years, if then. The United States plans to deploy only 44 interceptors capable of engaging ICBMs in 2017.

Russia could cut its strategic missile force by 50 percent or even 75 percent and still easily overwhelm those interceptors. Until the gap between strategic offense and defense narrows considerably, a U.S. political commitment not to target Russian missiles, coupled with transparency on missile defense plans, should suffice.

Domestic motives may lie behind the Russian position. President Vladimir Putin may see political value in scratchy relations with the United States. Similarly, the Russian Ministry of Defense may hope that keeping alive tensions over missile defense will produce greater resources for military modernization.

If the Russians want to continue the argument over missile defense, they can offer various pretexts. But that should not obscure the main point: their assertion that missile defenses, specifically SM-3 IIB interceptors, will threaten Russian ICBMs and thereby undermine the strategic balance now has no substantive basis.

So is Moscow prepared to engage in a serious way with Washington and NATO to settle the missile defense question and pursue a cooperative approach?

Russian officials have begun to offer a more nuanced reaction to Hagel’s announcement and, on Monday, he and the Russian defense minister, Sergei Shoigu, agreed to resume consultations on missile defense. This is good news.

The question now is whether Moscow can find a way to say yes. Or will it instead seek an excuse to keep the fight going? President Putin, over to you.