A plurality of Americans now believe that it was a mistake to send soldiers to fight a war in Afghanistan after the terrorist attacks of Sept. 11, 2001, marking the first time in the war's history that fewer Americans supported the combat effort than opposed it.

Forty-nine percent of Americans now believe the war was not ours to fight, according to a new Gallup poll, compared to 48 percent who still believe chasing the country's al Qaeda cells was the right thing to do. The margin is slight, but it marks a steep decline in those favoring the intervention over the years, which once registered at a high of 93 percent in early 2002.

But it has taken Americans longer to turn on its longest war than any other fought since the Korean War. It took the country only six months to sour on that campaign, in part due to Chinese intervention in North Korea that helped create a protracted quagmire.

Similarly, Americans turned against the Vietnam War relatively quickly. About a quarter of Americans told Gallup they opposed what Lyndon Johnson once dubbed "that bitch of a war" when the polling service first gauged public opinion in the early summer of 1965, while 6 in 10 supported it. By October 1967, 47 percent saw involvement there as a mistake, compared to 44 percent who supported it.

Remember Iraq? Just fifteen months passed from the start of that war in March 2003 before a majority of Americans--54 percent--viewed it as a mistake.

Part of the reason why it took so long for Americans to develop negative views of the war in Afghanistan is because so few opposed it at the outset. When the war started in those early, post-9/11 days, only 9 percent thought military intervention was a mistake. Between one in five and a quarter of Americans opposed the nation's other modern wars when they began.

The results are sure to rile defense hawks like Sen. John McCain, an Arizona Republican, who has expressed concern that the U.S. is turning inward and shirking its exceptionalistic duties as an international force for good and democratic order.

By a large margin, Democrats and "Democratic leaners" are more likely to view the Afghan war as a mistake. Fifty-nine percent now say putting troops on the ground in 2001 was wrong. Just 36 percent of Republicans think so.

Gallup's telephone interviews were conducted from Feb. 6-9 among a random sample of 1,023 adults. The poll's margin of error is plus or minus 4 percentage points.