Americans' views are now split down the middle, with 49 percent saying the involvement there was a mistake and 48 percent saying it was not, Xinhua cited from Gallup's Feb 6-9 World Affairs survey.
Gallup first asked Americans about US intervention in Afghanistan in November 2001, just after the US sent its military into the country after the Sep 11 terror attacks on New York and Washington DC that killed nearly 3,000 people.
The US invaded Afghanistan to punish the Taliban for harbouring al-Qaeda
The US invaded Afghanistan to punish the Taliban for harbouring Al Qaeda terrorists responsible for the attacks.
At that point, fewer than one in 10 Americans said US involvement there was a mistake -- the most of any war since Gallup first asked the question during the Korean War in 1950.
"Clearly, in the turbulent atmosphere and general 'rally effect' environment that followed 9/11, Americans were overwhelmingly supportive of the decision to send the US military to Afghanistan," Gallup said in a report that accompanied the survey.
Americans' perceptions that US involvement in Afghanistan was a mistake rose as the war continued, although there were some ups and downs over the years.
Those believing the war was a mistake reached 25 percent in 2004, and surpassed 30 percent for the first time in 2008, and 40 percent in 2010, according to Gallup.
Still, the more than 12-year span during which less than half of Americans thought the US made a mistake in entering Afghanistan has been remarkably long, relative to past US interventions, Gallup found.
It took only a year and three months from March 2003 start of the Iraq war for a plurality of Americans to first say involvement there was a mistake.
Americans' opinion about the Iraq war fluctuated until late 2005, after which they were more consistently negative, Gallup found.
Afghanistan has become America's longest war, stretching over 12 years since US military forces were first sent in 2001, with well over 35,000 troops still there.
The Obama administration plans to draw down the number of troops in Afghanistan significantly by the end of this year. Once that happens, Americans' assessment of whether the intervention was a mistake will largely depend on the political course Afghanistan takes, including whether terrorist cells are able to regroup there, Gallup said.