President Barack Obama will announce in his State of the Union address that 34,000 troops - about half the US force in Afghanistan - will withdraw by early 2014, a senior administration official says.
The decision brings the United States one step closer to wrapping up the unpopular and costly 11-year-old war but also appeared to give the White House time and flexibility before it answers bigger questions about America's exit strategy.
This includes the size of the US force that Obama will keep in Afghanistan once the NATO mission is completed and the war is declared formally over at the end of 2014.
Obama also must decide how large an Afghan force to finance, and for how long, as his allies in Congress press to keep them at their maximum strength.
The Obama administration official, speaking on condition of anonymity, said no further details would be provided however in this evening's address, due to begin at 9PM EST.
"The president will not be making any further announcements about troop numbers tonight, nor has he made any decisions beyond the one he is announcing," the official said.
The official promised further US troop reductions through the end of 2014 "as Afghans take full responsibility for their security".
The announcement comes a month after Obama and Afghan President Hamid Karzai agreed in Washington on a plan to slightly speed up the handover of combat operations in Afghanistan, with Afghan forces taking the lead role throughout the country this spring.
How the Afghan forces fare in this leading role has yet to be seen. Although US military officials express confidence in growing Afghan capabilities, Afghan forces remain highly dependent on US support.
Another US official, also speaking on condition of anonymity, said US commanders would have the flexibility to decide when to withdraw the 34,000 troops, as long as they were out by early next year, meaning the bulk of them could stay through this year's peak fighting months.
Jeffrey Dressler of the Institute for the Study of War, a Washington think-tank, said that flexibility would allow the military to "focus on the fight at hand" during the summer and early fall.
Still, he described the removal 34,000 troops as a "tall task" that comes at a challenging time for Afghan forces.
"The real question now is what the post-2014 presence will be, which in my mind, is a far more important question," Dressler said.
Previous discussions at the White House focused on a range of options of between 3000 and 9000 troops, with military commanders most comfortable with the higher-end figures.
It was unclear when Obama might make a decision.
Bruce Riedel, who chaired Obama's 2009 review of Afghan policy and is now at the Brookings Institution think-tank in Washington, said a crucial factor will be the extent to which al Qaeda's core leadership in Pakistan continues to degrade.
A stronger al Qaeda in neighbouring Pakistan means the United States would need a more robust counter-terrorism presence, for example, he said.
"The president is rightly not making that decision now when he doesn't have to," said Riedel, who heads the Intelligence Project at Brookings.
"He should wait and see how much success we have against (al Qaeda core)."