When it comes to supporting war , the American public has a very short attention span.

For every major military conflict since World War II, public support has waned as the combat operations  dragged on.

After combat troops hit the ground in Vietnam, it took three years for the majority of Americans to oppose the Vietnam War.  During the Korean War, initially support plummeted after a year and a half. For the war in Afghanistan, it took eight years.  And slightly more than a year after troops were deployed to Iraq in 2003, more than 50 percent of the public said the war was not worth fighting.

“These are great weights upon a country to be fighting a war of this sort,” said Steve Hess, a senior fellow emeritus at the Brookings Institution. “Not only in terms of life but in terms of treasure these are very costly events.”

While support quickly tapered for the most recent military conflicts , the public widely supported World War II throughout the nearly five years that the United States was at war, said Adam Berinsky, an associate professor of  political science at MIT, who has extensively studied public opinion of wars.

Berinsky said opinion polls during the second world war not once asked respondents if the war was a mistake, a question that was asked often during the Vietnam, Iraq and Afghanistan wars.

“The kinds of questions that [pollsters] ask is really a good indicator of what they are expecting,” Berinsky said. “The fact they never asked ‘was it a mistake’ is extremely telling.”

In 1945, at the height of the war and when public polling was in its infancy, the American Institute of Public Opinion found that 75 percent of respondents favored continuing to fight until the German army was “completely defeated,” even if Adolf Hitler offered to make peace, Berinksy said in a 2006 article .

Hess, who was a staff member or adviser to four presidents, said the country so supported World War II because the reason for going to war seemed clear.

“[World War II] was considered a just war,” Hess said. “The populous on the home front were entirely in favor of what was going on. The people that went there went there because they felt they had to and wanted to and should have been there.”

The same was not true of the Korean War.

“That is entirely different from subsequent wars, which were considered iffy as to whether the country was considered to be on the right course,” Hess said.

According to Gallup polls , nearly eight out of 10 Americans supported former President Harry Truman’s decision to send troops to Korea in 1950 shortly after World War II concluded.

But barely one year later, nearly half of the respondents said it was a mistake. A similar story played out in how the public reacted to the Vietnam War.

While broad majorities initially supported using military force in Vietnam, that support quickly evaporated as hundreds of thousands of men were drafted to fight and tens of thousands of them never returned home.

“Support for the war eroded over time,” said Carroll Doherty, the associate director of the Pew Research Center. “There was broad support for the war in the early- to mid-1960s, and that actually was sustained for longer than people might remember or realize, but eventually the public turned against it.”

According to USA Today/CNN Gallup polls , the first time the majority of Americans said U.S. military action in Vietnam was a “mistake” was in August 1968, three years after U.S. combat troops landed in the Asian jungle.

By the end of the decade, less than one-third of the population said the war was not a mistake.

That downward trend of approval never reversed itself. In a similar poll in November 2000, 69 percent of respondents said that looking back, it was a mistake to send troops to Vietnam.

Support for the War in Iraq has followed a similar trajectory. At the onset of war in 2003, 70 percent  of respondents in an ABC/Washington Post poll said it was worth fighting, compared with 27 percent who said it was not.

But since troops hit the ground, support for the war has continually declined. Less than a year later, support dipped to less than 50 percent for the first time when, in February 2004, only 48 percent of respondents said the war was worth fighting.

Since 2007, the percentage of people who say the Iraq War is worth fighting has not broken 40 percent. The latest ABC/Washington Post poll on Nov. 3 showed that only a third of the country still supports military efforts in Iraq, while 62 percent said the war was not worth fighting.

Despite the high unpopularity of both the Iraq and Vietnam wars, the two conflicts have had drastically different impacts on America.

“Vietnam was in many ways a very destructive war to American society,” Hess said. “Vietnam in some ways also became a class war. There were a disproportionate number of working-class people or minorities being drafted. I don’t think that is true in Iraq and Afghanistan.”