WASHINGTON - President Barack Obama is taking the first major step in his push toward a nuclear-free world, returning to Prague to sign the kind of arms-reduction treaty with Russia unseen for nearly two decades.

The deal goes beyond modest arsenal reductions, offering Obama a chance to repair soured relations with Moscow and pursue more dramatic cuts in global nuclear weapon stockpiles.

The new treaty, to be signed today by Obama and Russian President Dmitry Medvedev, will shrink both nations' arsenals of strategic nuclear warheads to 1,550 over seven years, about a third less than the 2,200 currently permitted. It was a year ago nearly to the day, also in Prague, that Obama outlined his agenda to reduce the role of nuclear weapons in U.S. security strategy, with a long-range goal of eliminating nuclear arms.

The agreement - "new START," as it is known - is clear evidence of an improved U.S.-Russian relationship that had fallen to such a low in recent years that some worried about a second Cold War, with disputes over U.S. missile defense plans, Moscow's 2008 invasion of Georgia and NATO expansion to Russia's doorstep. Under Obama, Russian cooperation on key priorities, from helping to prevent a nuclear-armed Iran to opening supply routes for the U.S. military into Afghanistan and agreeing to new arms reductions, has increased - though not by a huge amount.

Because of arcane warhead-counting rules involving delivery vehicles, the real reductions under the agreement could be far less than the advertised numbers. Regardless, the allowed stockpiles still leave plenty for global annihilation.

And, proof of continuing bilateral distrust, the process of achieving it was far more difficult than the Obama administration expected when the negotiations were inaugurated last April by Obama and Medvedev. Instead of an easy lift to be completed by December, when its predecessor, the 1991 Strategic Arms Reduction Treaty, expired, intense wrangling extended the talks by more than three months. Complications arose from disagreements over a new verification regime and other factors.

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