The last of the 33,000 "surge" troops ordered into Afghanistan by President Barack Obama more than three years ago have withdrawn from the country, returning the U.S. presence to pre-surge levels.
U.S. Defense Secretary Leon Panetta said the surge had accomplished "its objectives of reversing the Taliban momentum on the battlefield and dramatically [increasing] the size and capability of the Afghan national security forces."
Calling it a "very important milestone," Panetta said he believed the United States was on track to accomplishing its goals in Afghanistan.
Panetta said the top U.S. commander in Afghanistan, General John Allen, "is saying that the force he has put in place is sufficient to accomplish that mission."
The withdrawal, which leaves 68,000 American forces in the warzone, comes as the security transition to Afghan forces is in trouble, threatened by a spike in so-called "insider attacks" in which Afghan army and police troops, or insurgents dressed in their uniforms, have been attacking and killing U.S. and Nato forces.
The attacks have called into question the core strategy that relies on Nato troops working shoulder to shoulder with Afghans, training them to take over the security of their own country so the United States and its allies can leave at the end of 2014 as planned.
The number of U.S. forces there peaked at about 101,000 last year, and they have been coming out slowly over the past several months.
The surge was aimed at beating back the Taliban to give the Afghan government and its security forces the time and space to take hold. The key goal was to ensure the Taliban did not regain a foothold in the country that could allow it once again to become a safe haven for terror groups. And there was hope that Taliban members would be willing to come to the peace table.
Military commanders say the war strategy is on track and that they have made broad gains against the Taliban, wresting control of areas where the insurgents once had strong footholds. Panetta has characterised the insider attacks as the last gasp of a desperate insurgency.
But other top military leaders, including U.S. General Martin Dempsey, Chairman of the Joint Chiefs of Staff, are worried about the impact of the attacks on the troops. Dempsey called them a "very serious threat" to the war campaign and has declared that "something has to change."