In a newly declassified letter provided to The Associated Press, Gates wrote that he and the top U.S. military leadership have concluded that Russia would not be able to achieve "militarily significant cheating" under the New START treaty, even if it tried. He wrote that the Obama administration expects Russia to fully adhere to the treaty's limits.
If Russia were to exceed those limits, the Pentagon could respond by putting its doomsday submarines and bombers on higher alert and arming them with extra nuclear warheads, Gates wrote.
The "survivable and flexible" U.S. offensive nuclear arsenal will "help deter any future Russian leaders from cheating or breakout from the treaty, should they ever have such an inclination," Gates wrote.
Some Republicans have raised doubts about the treaty, questioning whether Russian compliance can be ensured. The Obama administration has made a strong pitch for Senate ratification, arguing that it serves America's national security interests.
In remarks Wednesday at the Council on Foreign Relations, Secretary of State Hillary Rodham Clinton said that the debate over the treaty has become partisan.
"It's a political issue," Clinton said. "I wish it weren't," since previous arms treaties have passed the Senate by wide margins.
"I hope that at the end of the day the Senate will say, you know, some things should just be beyond any kind of election or partisan calculation," she said.
The pact would shrink the limit on strategic warheads to 1,550 for each country, down about a third from the current ceiling of 2,200. A previous treaty on strategic arms limits expired last December.
Gates' letter to the chairman of the Senate Foreign Relations Committee, Sen. John Kerry, D-Mass., was intended to allay concerns raised by Sen. John McCain at a July 20 congressional hearing. At that hearing, the Arizona Republican responded incredulously to Pentagon statements that even large-scale Russian cheating on New START was of little military concern.
Kerry's panel is scheduled to meet next week to decide whether to send the treaty to the Senate floor for a vote.
The letter was distributed Wednesday to other members of Kerry's committee and other lawmakers. It originally was classified, but was declassified by the Pentagon after one paragraph was blacked out.