PRAGUE — Casting aside years of rancor, President Barack Obama and Russian President Dmitry Medvedev on Thursday signed the biggest nuclear arms pact in a generation, lacing the moment with new warnings of sanctions for an intransigent Iran.
The treaty, sealed after months of halting negotiation, is significant not just for what it does but for what it symbolizes: a fresh start for the United States and Russia, and evidence to a watching world that nuclear disarmament is more than a goal.
The pact commits their nations to slash the number of strategic nuclear warheads by one-third and more than halve the number of missiles, submarines and bombers carrying them.
That still leaves the two countries with enough nuclear firepower to ensure mutual destruction several times over, but the move sets a foundation for deeper reductions, which both sides are already pursuing.
"It sends a signal around the world that the United States and Russia are prepared to once again take leadership," Obama said moments after he and Medvedev signed the treaty in a gleaming, ornate hall in the Czech Republic's presidential castle.
Said the Russian president: "The entire world community has won."
The pact will shrink the limit of nuclear warheads to 1,550 per country over seven years, about a third less than the 2,200 currently permitted.
Looming over the celebration was Iran, which in the face of international pressures continues to assert that its uranium enrichment program is for peaceful purposes, not for weapons as suspected. Six powers — the U.S., Russia, Britain, France, Germany and now China — are in talks in New York about a fourth set of United Nations sanctions to pressure Iran into compliance.
"We cannot turn a blind eye to this," Medvedev said in a show of solidarity. But he said he was frank with Obama about how far Russia was willing to go, favoring only what he called "smart" sanctions that might have hope of changing behavior.
Russia's Deputy Foreign Minister Sergei Ryabkov later elaborated by saying, for example, that Russia would not endorse a total embargo on the delivery of refined petroleum products into Iran. Such products might be targeted in other ways, or sanctions on Iran's energy sector might be avoided altogether to avoid running into deal-breaking opposition from Russia or China.
The nuclear arms pact now faces a ratification vote in the Russian legislature and the U.S. Senate. At home, Obama's team is struggling to get the necessary votes, and the president himself is directly involved. He said he was confident that Democrats and Republicans would see that the treaty protects U.S. interests — an upbeat view of bipartisanship in a town where it's been scarce.
"I feel confident that we are going to be able to get it ratified," Obama said.
But prospects of the treaty's ratification are still uncertain. Democratic officials said they hoped the treaty could be ratified by the end of the year, but that the timing of the debate would depend upon the submission of technical documents accompanying the treaty.
The treaty's ratification also would depend on Republican support, which lawmakers were reluctant to extend just yet. While Democrats hold a majority in the Senate, they are one vote shy of the 60 votes that are often needed to overcome procedural hurdles during debate. And 67 votes are needed to ratify the treaty.
Negotiations between the U.S. and Russia got bogged down in disputes, including Russia's objection to U.S. missile defense plans for Europe. The Kremlin is still concerned about the plan but sought to tamp down talk it would withdraw from the new treaty if there is a buildup in the missile defense system. Russia codified its option to withdraw in a statement in connection with the treaty.
Obama said the treaty itself built trust that would help in solving any differences on the issue. Responded Medvedev: "I am an optimist as well as my American colleague. I believe that we will be able to reach a compromise."
Beyond slashing nuclear arsenals, the U.S. sees the new "START" treaty, as it is known, as a key part of efforts to reset ties with Russia, badly strained under the Bush administration, and engage Moscow more in dealing with global challenges, including the nuclear arsenal of North Korea and nuclear ambitions of Iran.