Since Barack Obama entered the White House, the number of US troops in Afghanistan has nearly tripled.

But after nearly 10 years of war, the US president plans to unveil plans on Wednesday for the start of a withdrawal of the nearly 100,000 American forces on the ground.

Here is a summary of how the conflict has evolved:

-- WAR ON TERROR: 2001

Less than a month after the September 11 attacks, then president George W. Bush -- declaring a "war on terror" -- launches an assault on Afghanistan to strike at Al-Qaeda, and to topple the Taliban regime for giving safe harbor to Osama bin Laden's network.

The Americans rely on a small force of CIA paramilitary teams, special operations forces and US warplanes to back up the Taliban's Afghan rivals -- the Northern Alliance.

The Taliban fall from power within days as Al-Qaeda training bases are knocked out, but bin Laden and his associates flee over the border into Pakistan. About 1,000 US Marines deploy later in November, the first substantial ground forces that later rise to about about 10,000 in 2002.

-- FORGOTTEN WAR: 2002-2007

The US focus shifts to Iraq, as Bush orders an attack on Iraq in 2003 to "disarm" Saddam Hussein's regime. US forces struggle against an Iraqi insurgency and spiraling sectarian violence, overshadowing the war in Afghanistan.

The Taliban and other Islamist militants regroup and re-emerge by 2005, exploiting sanctuaries in neighboring Pakistan while staking out strongholds along the eastern border and in the mainly Pashtun south. US troop levels hover around 20,000, rising to 27,000 in February 2007.

"In Afghanistan, we do what we can," Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said in 2007. "In Iraq, we do what we must."

-- WAKE UP CALL: 2008

With Taliban attacks on the rise, the Bush administration faces mounting criticism that Afghanistan has been neglected. Both presidential candidates promise to send more troops, with then senator Barack Obama arguing that Iraq has served as a "distraction."

The US commander, General David McKiernan, appeals for major reinforcements, advocating a counter-insurgency strategy designed to win over Afghans instead of merely targeting Taliban forces. At the end of his term, Bush orders thousands of troops to Afghanistan, but far less than the 30,000 requested by McKiernan.

-- OBAMA'S WAR: 2009

During the first months of his presidency, Obama decides to double the number of troops on the ground, with the force rising to 68,000, while unveiling plans to end the Iraq mission. He says the aim of the war is to "disrupt, dismantle and defeat" Al-Qaeda.

After a protracted debate inside the administration that pits US military commanders against advisers in the White House, Obama announces in December a "surge" of 30,000 additional troops, bringing the US contingent to nearly 100,000 along with nearly 50,000 allied troops.

The strategy focuses on securing key towns in the Taliban's southern strongholds while building up Afghan forces. At the same time, the CIA expands a covert bombing campaign against Al-Qaeda and Taliban leaders in neighboring Pakistan, using unmanned aircraft.

Along with the troop buildup, Obama sets a deadline of July 2011 to start the gradual withdrawal of US forces, but the pace of the drawdown remains unclear.

-- THE WAY OUT: 2014

At a November 2010 summit, NATO backs a plan for Afghan forces to take over security by the end of 2014, with the transition starting in mid-2011.

On May 2, US Navy SEALs kill Al-Qaeda founder Osama bin Laden in a raid on his Pakistani hideout, prompting calls for a faster troop withdrawal from Afghanistan.

As Obama prepares to outline the blueprint for a US drawdown, senior officers, White House aides and lawmakers debate the pace and scope of the pullout.

Military leaders favor a modest withdrawal to cement "fragile" progress and to buy time for possible peace talks. Some White House advisers and members of Congress argue for a more dramatic drawdown -- citing the death of bin Laden, fiscal pressures and waning public support.

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