The last of the 33,000 "surge" troops that had been ordered into Afghanistan by Barack Obama, the US president, in 2009 to help push back the Taliban have now been withdrawn from the country.
US presence has returned to pre-surge levels, a senior US defence official said on Friday.
The surge in American troops was designed to push back the Taliban and create space for NATO forces to build the Afghan army to a point where it could take over Afghanistan's security, allowing for an eventual Western drawdown.
The completion of the withdrawal had been expected by the end of September.
Obama has trumpeted ending the war in Iraq and winding down the war in Afghanistan as he seeks re-election ahead of the November 6 polls.
The return of US forces to pre-surge levels comes as NATO commanders wrestle with an upswing in so-called "insider attacks" of Afghan forces turning their guns on Western troops.
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NATO announced this week it was scaling back some joint operations with Afghan troops as a result, raising questions about Obama's plan to stabilise the country ahead of the expected withdrawal of most combat troops by the end of 2014.
Speaking to Al Jazeera, Kurt Volker, a former US ambassador to NATO, said the 2014 withdrawal deadline set by international forces has had a psychological effect on people in the country, with many people expected the return of the Taliban.
"I don't think anybody can feel that we've created sustainability and success in Afghanistan. The Taliban still feels
"The Afghan security forces are still to be trained up, and as we continue to reduce the international military presence there, we really don't know if the Afghan security forces are going to be capable of maintaining security and order in the country or whether we are going to see the return of the Taliban ..."