President Barack Obama announced that 34,000 troops would be pulled out of Afghanistan in the next year, and vowed that by the end of 2014 the US war in the country would be over.
"After a decade of grinding war, our brave men and women in uniform are coming home," Obama said in his State of the Union address, winning applause and a standing ovation from lawmakers.
The long-awaited move effectively halves the size of the current 66,000-strong US force in Afghanistan, ahead of a final withdrawal of most foreign combat troops by the end of 2014.
"We can say with confidence that America will complete its mission in Afghanistan, and achieve our objective of defeating the core of Al-Qaeda," Obama said in praising the sacrifices made since the 2001 US-led invasion of Afghanistan toppled its Taliban rulers.
"Tonight, I can announce that over the next year, another 34,000 American troops will come home from Afghanistan. This drawdown will continue. And by the end of next year, our war in Afghanistan will be over."
There were no immediate details of how quickly the drawdown would take place. But a senior Pentagon official told AFP earlier that it would be tied to the fighting season in Afghanistan, which runs into the fall.
"The commanders will have discretion on pace and focus will be (on) keeping as many forces in play until after the fighting season," the official said, adding that outgoing US Defense Secretary Leon Panetta supports the plan.
A senior US official said Obama had telephoned Afghan President Hamid Karzai, British Prime Minister David Cameron and German Chancellor Angela Merkel to inform them of his decision.
Debate is also taking place within the administration on the size of a residual force -- to train Afghan soldiers and to conduct counterterrorism missions -- that will remain behind after the formal withdrawal.
The administration was "negotiating an agreement with the Afghan government that focuses on two missions: training and equipping Afghan forces so that the country does not again slip into chaos, and counterterrorism efforts that allow us to pursue the remnants of Al-Qaeda and their affiliates," Obama said.
Last month, US officials suggested it was theoretically possible that Washington would leave no troops in the country, though some observers saw that move as a negotiating tactic with Karzai in town.
The senior official said that Washington remained committed to a long-term strategic partnership with Afghanistan, and reiterated that talks on a bilateral security agreement were still taking place.
Afghanistan has committed to taking full responsibility for its own security after US forces leave, and the White House said Afghan security forces now count 352,000 troops, thanks to a broad NATO training effort.
NATO says it will no longer lead combat operations in the next two years, but will provide support to Afghan soldiers.