WASHINGTON (AP) — 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.