The new US-Russian arms control treaty is more important for the diplomatic bargain it seals with a restive Russia than for the limits it places on weapons that neither side was likely to use, treaty or no treaty.

Despite scepticism from Republican opponents who worry that the United States would be purposely fraying its nuclear advantage, the Obama administration considers the pact a disarmament bargain because it probably will help cinch Russian cooperation with an American plan to protect Europe with an anti-missile shield arrayed against Iran.

The Senate voted 71-26 to ratify the treaty, and Russian legislative approval is expected quickly. The Senate approval was a clear victory for the White House after weeks in which it seemed doubtful that President Barack Obama could muster enough Republican votes. Obama had called approval of the treaty his highest foreign policy priority this year.

“This is the most significant arms control agreement in nearly two decades,” Obama said shortly after the Senate approved the treaty on Wednesday. He asserted that the treaty “will make us safer” and point out that it would allow US inspectors to return to Russian nuclear bases.

The pact, called New START, is the centrepiece of a mutual US-Russian effort to repair relations badly damaged during the latter years of the George W. Bush administration.

Obama and Russian President Dmitry Medvedev signed the treaty in April with bonhomie intended to show that the pact was about making things right with Moscow as well as reducing warheads. Russian leaders let it be known they considered the treaty a test of Obama’s sincerity and clout.

The New START treaty replaces the expired Strategic Arms Reduction Treaty of 1991. It sets a limit of 1,550 strategic nuclear warheads for each side, down from 2,200 under a 2002 deal. The pact also re-establishes anti-cheating procedures that were not written into the 2002 accord, thus providing the most comprehensive and substantial arms control agreement since the 1991 treaty.

But that ambitious nuclear downscaling was jeopardised when Obama won NATO support in November to build a missile shield over Europe. The commitment to erect the shield, ostensibly aimed at Iran, remained one of the major irritants with Russia until Obama scrapped plans to stage parts of it at Russia’s doorstep.

His reworked missile shield plan got a polite but noncommittal reception from Moscow, and Obama’s advisers breathed a cautious sigh of relief. A public fight with Russia over missile defence might have sunk the new treaty with just weeks to go before the close of the current congressional session.

With Senate approval, the administration can fully exhale. Carrying the treaty over into 2011 would have made it harder to pass, and rubbed salt in already-sensitive Russian irritation at the slow pace of US approval and at Republican suggestions that parts of the treaty might have to be reworked.

New START also is meant as an example to other nations that might want to build up arsenals or resist US entreaties to reduce existing stockpiles. Although the treaty leaves both the United States and Russia with plenty of firepower to annihilate one another, the large reductions it imposes are supposed to bolster US credibility when it argues that the world should work toward a day when nuclear weapons are eradicated.

The passing of the Cold War, the rise of radical Islamic extremism and the emergence of sophisticated terrorist networks have changed the nuclear equation.