For the first time since the U.S. sent troops into Afghanistan in 2001, more Americans agree the intervention was a mistake than disagree.

In a new Gallup poll, 49 percent said the United States made a mistake sending troops into Afghanistan after the Sept. 11 terrorist attacks, versus 48 percent who said it was not a mistake.

Although the margin is statistically insignificant, it's the first time in the 12 years Gallup has polled on this question that more people said the intervention was a mistake than not. The last such poll in March 2013 found that 51 percent of Americans still believed the war was not a mistake.

But the past year has seen the unraveling of much of the U.S.-led effort in Afghanistan, which has cost the lives of more than 2,100 American troops and more than 1,000 from allied nations. The Obama administration's policy has been adrift since Afghan President Hamid Karzai refused to sign a bilateral security agreement, increasing the likelihood the Taliban may regain power if a full withdrawal takes place at the end of the year.

Meanwhile, American troops continue to die. Two U.S. soldiers were killed Feb. 12 by two men wearing Afghan army uniforms at a base in Kapisa province -- the latest in what has become a common phenomenon of insider attacks on coalition troops by Afghans posing as allies.

The Feb. 6-9 poll of 1,023 adults had margin of error of plus or minus 4 percentage points.