The phased drawdown is likely to start with 5000 troops recalled this northern summer and an additional 5000 by winter or spring 2012, according to a senior US defence official. Obama is also weighing a timetable for bringing home the 20,000 other troops he ordered to Afghanistan as part of his December 2009 decision to send reinforcements to reverse the Taliban's battlefield momentum.
The withdrawals would put the US on a path toward giving Afghans control of their security by 2014 and ultimately shifting the US military from a combat role to a mission focused on training and supporting Afghan forces.
Obama is to address the nation from the White House tomorrow.
The president reached his decision a week after receiving a range of options from Gen. David Petraeus, the top US and NATO commander in Afghanistan. Obama informed his senior national security advisers, including outgoing Defencse Secretary Robert Gates and Secretary of State Hillary Rodham Clinton, of his plans during a White House meeting on Tuesday.
"The president is commander-in-chief," said spokesman Jay Carney. "He is in charge of this process, and he makes the decision."
The Obama administration has said its goal in continuing the Afghan war, now in its 10th year, is to blunt the Taliban insurgency and dismantle and defeat al-Qaeda, the terror network that used Afghanistan as a training ground for the September 11, 2001, attacks.
As of Tuesday, at least 1522 members of the US military had died in Afghanistan as a result of the US invasion of the country in late 2001, according to an Associated Press count.
Roughly 100,000 US troops are in the country, three times as many as when Obama took office. Even by drawing down the 30,000 surge forces, there will still be great uncertainty about how long the remaining 70,000 troops would stay there, although the US and its allies have set December 31, 2014, as a target date for ending the combat mission in Afghanistan.
A reduction this year totalling 10,000 troops would be the rough equivalent of two brigades, which are the main building blocks of an Army division. It's not clear whether Obama's decision would require the Pentagon to pull out two full brigades or, instead, a collection of smaller combat and support units with an equivalent number of troops.
If Obama were to leave the bulk of the 30,000 surge contingent in Afghanistan through 2012, he would be giving the military another fighting season - in addition to the one now under way - to further damage Taliban forces before a larger withdrawal got started. It also would buy more time for the Afghan army and police to grow in numbers and capability.
Under that scenario, the emphasis in US-led military operations is likely to shift away from troop-intensive counterinsurgency operations toward more narrowly focused counterterrorism operations, which focus on capturing and killing insurgents.
Afghan security forces and judicial institutions are expected to take up many aspects of the counterinsurgency fight by establishing the rule of law and respect for government institutions, US officials in Afghanistan said on Tuesday.
In recent speeches, Afghan President Hamid Karzai has criticised American forces, suggesting his ally is in danger of becoming an occupying force. He has even threatened action against international forces that conduct air strikes and has accused allies of undermining and corrupting his government.
Yet there are concerns in his country about the withdrawals. Some of the areas slated to transition to Afghan control have been struck by attacks in recent weeks despite assertions by Karzai that peace talks have started between the US, his Afghan government and Taliban emissaries. Publicly, the Taliban say there will be no negotiations until foreign troops leave Afghanistan.
The transition to full Afghan control will begin in earnest on July 20 in five provincial capital cities and two provinces. The provincial capitals identified for transition are Lashkar Gah in Helmand province, plus capitals from provinces in the west, east and north and most of Kabul, the nation's capital. The largely peaceful northern provinces of Bamyan and Panjshir will also start to transition to Afghan control.
Some US military commanders have favoured a more gradual reduction in troops than Obama is expected to announce tomorrow, arguing that too fast a withdrawal could undermine the fragile security gains.
But other advisers have backed a more significant withdrawal that starts in July and proceeds steadily through the following months. That camp believes the slow yet steady improvements in security, combined with the death of Osama bin Laden and US success in dismantling much of the al-Qaeda network in the country, give the president an opportunity to make larger reductions this year.
Obama has previously said he favors a "significant" withdrawal beginning in July, his self-imposed deadline for starting to bring U.S. troops home. Aides, however, have never quantified that statement.
Pressure for a substantial withdrawal has been mounting from the public and Congress. Even Gates, who has said he favored a "modest" withdrawal, said on Tuesday that Obama's decision needed to incorporate domestic concerns about the war.
"It goes without saying that there are a lot of reservations in the Congress about the war in Afghanistan and our level of commitment. There are concerns among the American people who are tired of a decade of war," Gates said during a news conference at the State Department on Tuesday.
According to an Associated Press-GfK poll last month, 80 percent of Americans say they approve of Obama's decision to begin withdrawal of combat troops in July and end US combat operations in Afghanistan by 2014. Just 15 percent disapprove.
On Capitol Hill, even the more conservative members of his party, such as Sen. Joe Manchin of West Virginia, are pressing for significant cuts and a shift in mission.
"The question the president faces - we all face - is quite simple: Will we choose to rebuild America or Afghanistan? In light of our nation's fiscal peril, we cannot do both," Manchin said on Tuesday.
Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee, said that improved conditions in Afghanistan would permit Obama to withdraw at least 15,000 troops by the end of the year.
Obama aides have sidestepped questions about what role the cost of the Afghan war played in Obama's decision, saying only that the president was focused on meeting the goal of transferring security by 2014.
Following the announcement on the drawdown, Obama will visit troops on Thursday at Fort Drum, the upstate New York Army post that is home to the 10th Mountain Division, one of the most frequently deployed divisions to Afghanistan.
