President Barack Obama has threatened to withdraw all American forces from Afghanistan if a new security agreement is not signed by the end of the year, but there is no legal reason the US has to resort to the "zero option" as administration officials have repeatedly claimed.
Legally, the 33,600 US forces still deployed are covered by an existing status-of-forces document that took effect shortly after the September 11, 2001 terror attacks and the start of America's engagement in Afghanistan.
The existing agreement has no expiration date and prevents US military personnel from being prosecuted under Afghan law a must-have for status of forces agreements the US signs with countries around the world.
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US officials privately acknowledge that there is no legal reason that would force Obama to withdraw all troops if the new security agreement is not signed by December 31 when the international combat mission ends.
Yet even though a full troop exodus is not the administration's preferred option, blunt rhetoric coming from US officials has continued to put the onus on Afghanistan: Sign the new bilateral security agreement (BSA) or every US service member will be forced to leave.
National Security Adviser Susan Rice conveyed that message to Karzai when she was in Afghanistan in November.
According to the White House account of their meeting, Rice told Karzai that "without a prompt signature, the US would have no choice but to initiate planning for a post-2014 future in which there would be no US or NATO troop presence in Afghanistan."
Last month, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told reporters travelling with him in Afghanistan that while the US remains committed to helping Afghanistan after this year, "I can't ask the young men and women to serve in a country without the protections afforded by a bilateral security agreement."
Daoud Yaqub, a former official at Afghanistan's National Security Council, said there is no legal foundation for that argument because the existing status-of-forces agreement would remain in effect.
"To state that there is no legal cover for US troops in Afghanistan beyond 2014 is not accurate, in my view," Yaqub said.
"Whoever is making such an assumption is not motivated by legal issues, but by other things, perhaps politics.