On Nov. 12, the day after President Bush addressed the U.N. General Assembly, national security adviser Condoleezza Rice and Secretary of State Colin L. Powell met with top Russian officials in a small, first-floor conference room at the Russian mission to the United Nations for a frank talk about the Anti-Ballistic Missile Treaty.

Rice did most of the talking, saying Russia would have to allow the United States to test missile defense systems without restrictions or the Bush administration would withdraw the United States from the ABM Treaty. "She was as blunt as she could be," one administration official said.

Russian Foreign Minister Igor Ivanov asked, "In your mind, what are the chances that we would accept a deal like that?"

Rice said she thought it was a good deal for Russia. Ivanov said nothing.

It was a key moment leading up to Bush's decision, expected to be announced today, to withdraw unilaterally from the ABM Treaty, defying the advice of allies and the wishes of Moscow so that the United States can pursue research on missile defense systems that Bush says are critical for guaranteeing the nation's safety.

Though administration officials have repeatedly threatened to pull out of the treaty, Bush's decision -- to exercise a clause allowing either side to withdraw from the treaty with six months' notification -- marks a historic turning point in arms control.

For three decades, the ABM Treaty has been a cornerstone of strategy between the world's two biggest nuclear powers. It was designed when those Cold War rivals saw the threat of mutual destruction as a guarantee of international safety, and codified limits on missile defenses as vital.

Russian leaders say the treaty is still needed. But the Bush administration says it is time to scrap the accord, rely on informal assurances and turn U.S. attention toward missile threats from smaller rogue states or groups. Bush has called the ABM Treaty, which prevents many forms of missile defense tests, a "relic" of the Cold War.

The Bush decision is a triumph for the administration's conservatives, who have wanted to scrap the treaty. But leading Democrats and some administration members such as Powell have said it would be better to get Russia to agree to modifications that would allow for missile defense tests.

"Ultimately, the gap was unbridgeable," said one administration official. "The Russians wanted a treaty. The administration didn't want a treaty." He added, "The administration wants maximum flexibility. The Russians wanted something that allowed them some oversight" of missile defense tests and deployment.

Bush phoned Russian President Vladimir Putin last Friday to tell him the United States would pull out in the coming days, as Putin must have anticipated. Administration officials said they expect Putin to issue a statement of regret, but not of outrage.

"There will be no hysterics," Mikhail Margelov, head of the foreign affairs committee of the Russian Federation Council, told the ITAR-Tass news agency.

U.S. officials said they expect Putin to say the U.S.-Russia relationship is strong enough to overcome this disagreement and to point to the agreement the two sides are nearing on deep cuts in strategic nuclear arsenals. U.S. officials expect Putin to propose targets for nuclear stockpiles that would largely overlap with numbers the administration has proposed.

On the ABM Treaty, the Russians "don't agree with it. But they are not going to say it's the end of the world," one senior administration official said.

Last month's Rice-Powell-Ivanov meeting was one of the last chances to salvage the ABM Treaty, though some critics of the administration say Bush's foreign policy team was never really committed to doing so.

Bush had proposed eliminating the treaty in a speech at The Citadel in September 1999 during his presidential campaign. Defense Secretary Donald H. Rumsfeld has called the treaty an obstacle. John Bolton, who as undersecretary for arms control and international security affairs is the State Department official responsible for negotiations, had criticized the treaty before joining the administration.

Nonetheless, the administration said it would try to obtain Russian agreement to amend or mutually put aside the treaty so that missile defense systems could be developed against common foes.

In early August, shortly after the Genoa summit between Bush and Putin, Bolton and Deputy Foreign Minister Georgy Mamedov met in Moscow. Mamedov suggested the two sides pick up where the Clinton administration left off, defining distinctions between theater and national missile defense, carving out areas for testing. Bolton told Mamedov the new president had a new policy.

In subsequent talks, Russia said it was willing to allow missile defense tests -- with conditions. When Putin and Bush met in Shanghai on Oct. 21 during the Asia Pacific Economic Cooperation meeting, Putin presented ideas that would have allowed missile tests for a certain period of time, an administration official said.

But an administration official said the Russian proposal "meant involvement . . . and a measure of approval of what they would agree to and not agree to at a very low level of detail. And that was not acceptable to us." Another official said Russia was never prepared to allow tests of space-based weapons, one of Rumsfeld's priorities.

Nonetheless, Putin's proposals gave Bush pause. Administration officials said Bush's talking points called for him to notify Putin during the Shanghai meeting that the U.S. planned to give its six-month notice in January if they did not reach agreement. But administration officials said Bush softened his message and did not give a date.

On Nov. 3, a week before the U.N. General Assembly meeting, Rumsfeld met in Moscow with Putin, who said he would not agree to give the latitude for missile testing that the administration wanted. Rumsfeld said that meant the United States would withdraw from the treaty, and he later asked Putin's security adviser whether Russia would prefer the withdrawal notification to be made before, during or after Putin's visit to the United States later that month.

Five days after the U.N. meetings, Bush and Putin met at Bush's ranch in Crawford, Tex., where Bush concluded the two would not reach any agreement on revising or setting aside the ABM Treaty.

As a result, Powell's trip to Moscow this week was designed in part to choreograph the U.S. decision and Russian reaction.

By the time Powell arrived in Moscow on Monday, few U.S. officials expected any change in Russia's position. Bush had phoned Putin with his decision. Instead, Powell pushed forward on the agreement to reduce stockpiles of nuclear weapons. That, U.S. officials reasoned, would give Putin something to boast about, help assuage Russia's military establishment and keep the two countries talking about cooperation on nuclear weapons. Bush has pledged to cut the U.S. arsenal from about 6,000 warheads to 1,700 to 2,200.

That strategy marked a change from an earlier administration posture. In July, Rice had said a deal on cutting arsenals would be part of a grand bargain on missile defenses and the ABM Treaty. Now that link has been severed. Even though Russia has not given its approval of U.S. missile defense tests, Powell last week said the two sides were close to a formal agreement on stockpiles that would include verification measures.

Even if the Russian reaction is muted, some Russia experts said Bush's decision could have unwelcome consequences. It could draw out hard-liners in the Russian military and political elite, who have been predicting for months that Putin's turn to the West would prove a mistake.

Sergei Markov, a political analyst, characterized the White House decision as "a slap in the face." He said, "Putin will have to send a signal to the U.S. that he is not going to give everything to the U.S. and get nothing in exchange."

Vladimir Lukin, vice speaker of the state Duma and a former Russian ambassador to the United States, said U.S. withdrawal from the ABM Treaty suggests that Russia's influence in Washington has waned with the success of the war in Afghanistan.

"We supported the U.S. unconditionally, we worked, we shared all sort of very sensitive data to do with combating terrorism," he said. "What happened after that is the moment we scored the victory, this line prevailed in the U.S.: 'Thanks, but in matters concerning both of us, we will be acting the way we want.' "

Bush's decision could again raiseamong European allies and Democratic critics the specter of American unilateralism, which has been less of an issue since the Sept. 11 attacks invigorated U.S. diplomacy.

Strobe Talbott, President Bill Clinton's deputy secretary of state and lead negotiator with Russia, said yesterday that European allies would wonder "whether the extraordinary pivot the administration did on Sept. 11 is going to be temporary and tactical or a real sea change." He added: "The way in which the administration handled the ABM Treaty issue will be seen today as an unwelcome indicator."

Though word of Bush's decision leaked on Tuesday, congressional leaders were not formally briefed by the White House until yesterday.

At a news conference after meeting with Bush at the White House, Senate Majority Leader Thomas A. Daschle (D-S.D.), said, "I think it's unfortunate that the Russians knew before the [congressional] leaders did." He added, "It's unfortunate that a matter of this import would not have been vetted more carefully, more completely and with greater care for U.S. foreign policy than this was."

LaFraniere reported from Moscow. Staff writers Helen Dewar and Mike Allen contributed to this report.

The ABM Treaty is a key issue for Secretary of State Colin L. Powell, right, during his meeting in Moscow with Russian Foreign Minister Igor Ivanov.