In a pivotal moment for US national security strategy, Obama also signaled in a 13-minute primetime speech on Wednesday that the United States would no longer try to build a "perfect" Afghanistan from a nation ravaged by generations of violence.
"Tonight, we take comfort in knowing that the tide of war is receding," Obama said in the East Room of the White House in an address blanketing US television networks at a time of rising discontent on the war.
"Even as there will be dark days ahead in Afghanistan, the light of a secure peace can be seen in the distance. These long wars will come to a responsible end," Obama said.
The president's speech came as domestic political support fades for the war following the killing of Al-Qaeda chief Osama bin Laden by Navy SEALS, and as Washington backs fragile Afghan reconciliation talks with the Taliban.
His decision on troop numbers amounted to a rejection of appeals from the Pentagon for a slower drawdown to safeguard gains against the Taliban and to allow a new counter-insurgency mission to unfold in eastern Afghanistan.