Asserting that the country that served as a launching pad for the Sept. 11, 2001 attacks no longer represented a terrorist threat to the United States, Mr. Obama, in remarks prepared for delivery at 8 p.m. from the East Room of the White House, announced plans to withdraw 10,000 troops from Afghanistan by the end of this year. The remaining 20,000 troops from the 2009 ?surge? of forces would leave by next summer, amounting to about a third of the 100,000 troops now in the country.

The troop reductions, which came after a short but fierce internal debate, are both deeper and faster than the recommendations made by Mr. Obama?s military commanders, and they come as the president faces relentless budget pressures, an increasingly restive Congress and American public and a re-election campaign next year.

The withdrawals would mark the start of a winddown of the military?s troop-intensive counterinsurgency strategy, which Mr. Obama signed on to 18 months ago after a painstaking review. Most American forces are expected to leave Afghanistan by 2014. Administration officials indicated that they now planned to place more emphasis on smaller, focused counterterrorism operations of the kind that killed Osama bin Laden in Pakistan last month, which the president cited as Exhibit A for a substantial American troop reduction.

Administration officials have said an intense campaign of drone strikes and other covert operations in Pakistan have crippled Al Qaeda?s original network in the region, leaving its leaders either dead or pinned down in the rugged border between Pakistan and Afghanistan. Of 30 top Qaeda leaders indentified by American intelligence, 20 have been killed in the last year and a half, the officials said.

But the withdrawal of the entire surge force by the end of next summer will significantly change the way that the United States wages war in Afghanistan, analysts said, suggesting that the administration may have concluded it can no longer achieve its loftiest ambitions for the nearly decade-long military campaign there.

Senior administration officials acknowledged as much during a conference call with reporters on Wednesday.

?We?re not trying to make Afghanistan a perfect place,? a senior administration official said. ?We?re not trying to pacify the entire country, or destroy every last vestige of the Taliban.?

Mr. Obama?s decision is a victory for Vice President Joseph R. Biden Jr., who has long argued for curtailing the military engagement in Afghanistan. But it is a setback for his top commander in Afghanistan, Gen. David H. Petraeus, who has been named director of the Central Intelligence Agency.

General Petraeus, who helped write the Army?s field manual on counterinsurgency policy, did not endorse the decision, said another official, though both Defense Secretary Robert M. Gates and Secretary of State Hillary Rodham Clinton both accepted it, albeit with reservations. General Petraeus had recommended limiting withdrawals to 5,000 troops this year and another 5,000 over the winter, deferring the drawdown of the rest of the surge force through next year?s fighting season.

He and other military commanders argued that the 18 months since Mr. Obama announced the troop increase did not allow for enough time for the Americans to consolidate the fragile gains that they had made in Helmand and other provinces. Troops have succeeded in clearing many towns and cities of insurgents, and then keeping them safe so that markets reopened and girls were able to go to school, for example.

But the effort to transfer responsibility for security to Afghan forces remains elusive because the Afghan troops are proving unprepared for the job.

While senior administration officials said Wednesday that the military campaign had made Afghans better able to govern themselves, they cited few specific initiatives that showed the Afghan government taking more responsibility for its citizens? security. Corruption in the government of President Hamid Karzai continues to be rampant, sapping the confidence of many Afghans.

Still, the growing disenchantment at home with the war, particularly given the ballooning national debt, the country?s slow economic recovery, and the whopping $120 billion price tag of the Afghan war this year alone, were all considerations weighed by the president in deciding it was time to begin disentangling the United States from its broad commitments in Afghanistan.

Even Republicans have been getting on the get-out-of-Afghanistan bandwagon. Republican presidential hopefuls like Mitt Romney and Jon M. Huntsman are demanding a swift withdrawal from Afghanistan, while Democrats on Capitol Hill and elsewhere complain that the cost of the war is siphoning money away from efforts to build roads and create jobs in the United States.

Representative Dana Rohrabacher , Republican of California, called on Mr. Obama to speed up the withdrawal on Wednesday. ?If we?re going to leave, we should leave,? he said in a statement. ?The centralized system of government foisted upon the Afghan people is not going to hold after we leave. So let?s quit prolonging the agony and the inevitable.?

The decision reflected a partial pivot by Mr. Obama himself, who campaigned for president on the argument that American resources were better spent in Afghanistan, from where the Sept. 11 attacks were planned, than in Iraq, a war he continues to characterize as one of choice.

Since then, Mr. Obama, pressed forcefully by Mr. Biden, has come to view Afghanistan as the far lesser threat to America than its neighbor next door, Pakistan.

Shanker contributed reporting