Updated

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 U.S. has to resort to the "zero option," as administration officials have repeatedly claimed.

Legally, the 33,600 U.S. forces still deployed are covered by an existing status-of-forces document that took effect shortly after 9/11 and the start of America's engagement in Afghanistan. The existing agreement has no expiration date and prevents U.S. military personnel from being prosecuted under Afghan law — a must-have for status-of-forces agreements the U.S. signs with countries around the world.

"Unless the Afghans or the United States cancel the existing SOFA, it remains in effect," said retired Col. Manuel Supervielle, who was the lead lawyer for U.S. forces in Afghanistan in 2005 and 2006, worked on such agreements for U.S. troops stationed across the globe and advised on the drafting of the current bilateral security agreement that Afghan President Hamid Karzai says he won't sign.

The new security agreement is likely to be addressed by U.S. Gen. Joseph Dunford, a top commander in Afghanistan, when he testifies Wednesday on Capitol Hill.

U.S. 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 Dec. 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 U.S. officials has continued to put the onus on Afghanistan: Sign the new bilateral security agreement or every U.S. 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 U.S. would have no choice but to initiate planning for a post-2014 future in which there would be no U.S. or NATO troop presence in Afghanistan."

Last month, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told reporters traveling with him in Afghanistan that while the U.S. 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 U.S. 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."

Both the existing status-of-forces agreement and the new bilateral agreement give the U.S. criminal jurisdiction over American personnel and troops accused of crimes while they are in Afghanistan, preventing them from being subject to Afghan prosecution.

Over the years, both the U.S. and Afghanistan have expressed a need for a more formal status-of-forces agreement to replace what currently is in the form of a diplomatic note. The two sides agreed on the language of the new bilateral security pact in November after about a year of tense on-again, off-again negotiations.

The agreement is set to remain in force until the end of 2024 and beyond, unless terminated by mutual agreement or by either party with two years' written notice. It pledges U.S. backing for Afghanistan and its security forces for many years to come and also allows the U.S. to use bases across the country.

There is more at stake. Other nations with troops in Afghanistan have said they won't stay without a U.S. presence and the exit of all international forces would jeopardize billions in foreign aid pledged to fund the Afghan security forces and help finance development in the impoverished country.

For now, the Obama administration continues to hold firm in its standoff with Karzai.

"The U.S. and the Afghan governments have long agreed on the need for any future U.S. military presence to be on a new basis, with a new invitation — the one now reflected in the draft BSA," Defense Department spokeswoman Elissa Smith said in a written statement. "As we have made clear, we need a willing and committed partner to pursue a post-2014 mission, and the conclusion of the BSA therefore remains a necessary prerequisite for any post-2014 U.S. military presence."

Obama last month ordered the Pentagon to accelerate planning for a full U.S. troop withdrawal by the end of this year. Still, he holds out hope that the agreement will be signed because the administration would like to leave up to 10,000 troops in Afghanistan to continue training, advising and assisting Afghan security forces and conducting counterterrorism missions.

Obama's order, following his first phone call with Karzai since June, appeared aimed at marginalizing the longtime Afghan leader's role in the high-stakes negotiations over the future of the lengthy American-led war. Karzai has deeply irritated Washington with anti-American rhetoric, as well as with his recent decision to release 65 prisoners over the objections of U.S. officials.

Karzai isn't budging either.

Karzai has ignored the recommendations of a council of more than 2,500 Afghan elders, who not only approved the agreement but urged him to sign it. Karzai also outlined new conditions for signing, saying the U.S. must curb night raids on Afghan homes and demonstrate a sincere commitment to help start stalled peace talks with the hard-line Taliban insurgents.

Moreover, he said the U.S. should wait to finalize the agreement with his successor. If there is no clear winner of the April 5 presidential election, there could be a second-round ballot. By some estimates, that could mean Afghanistan would not have a new president until late this year, giving the U.S. military even less time to implement plans for a post-2014 presence.