The Los Angeles Times reports that Panetta's decision was done to preempt any similar decisions from other NATO allies.
By announcing a specific timetable, US officials are hoping to head off a push by allies to pull out their forces more quickly. Public support for the war is falling in many countries, and with their economies struggling, governments are under pressure to trim their defense budgets.
With the expedited end to a combat role, US troops will turn primarily to training and advisory missions, similar to the way the withdrawal unfolded from Iraq, with the US focused on training Iraqi soldiers for more than a year before the last convoy left the country. Whether Afghan troops are ready to take on the central role is unclear – both the Army and police are "plagued by corruption, operational and personnel problems," according to the Los Angeles Times.
Afghan forces already have assumed control in Kabul, the capital, and some other areas, but those were already largely peaceful. The US and its allies retain military responsibility for the most violent parts of the country.
A senior Defense Department official traveling with Panetta said the US-led force "still needs to be there in robust fashion to back them up" until the end of 2014.