When it comes to war, the American public is remarkably fickle.

The responses of Americans to the Iraq and Afghanistan wars provide telling examples. In 2003, according to opinion polls, 72 percent of Americans thought going to war in Iraq was the right decision. By early 2013, support for that decision had declined to 41 percent. Similarly, in October 2001, when U.S. military action began in Afghanistan, it was backed by 90 percent of the American public. By December 2013, public approval of the Afghanistan war had dropped to only 17 percent.

In fact, this collapse of public support for once-popular wars is a long-term phenomenon. Although World War I preceded public opinion polling, observers reported considerable enthusiasm for US entry into that conflict in April 1917. But, after the war, the enthusiasm melted away. In 1937, when pollsters asked Americans whether the United States should participate in another war like the World War, 95 percent of the respondents said "No."

And so it went. When President Truman dispatched US troops to Korea in June 1950, 78 percent of Americans polled expressed their approval. By February 1952, according to polls, 50 percent of Americans believed that US entry into the Korean War had been a mistake. The same phenomenon occurred in connection with the Vietnam War. In August 1965, when Americans were asked if the US government had made "a mistake in sending troops to fight in Vietnam," 61 percent of them said "No." But by August 1968, support for the war had fallen to 35 percent, and by May 1971 it had dropped to 28 percent.

Of all America’s wars over the past century, only World War II has retained mass public approval. And this was a very unusual war – one involving a devastating military attack upon American soil, fiendish foes determined to conquer and enslave the world, and a clear-cut, total victory.

In almost all cases, though, Americans turned against wars they once supported. How should one explain this pattern of disillusionment?

The major reason appears to be the immense cost of war — in lives and resources. During the Korean and Vietnam wars, as the body bags and crippled veterans began coming back to the United States in large numbers, public support for the wars dwindled considerably. Although the Afghanistan and Iraq wars produced fewer American casualties, the economic costs have been immense. Two recent scholarly studies have estimated that these two wars will ultimately cost American taxpayers from $4 trillion to $6 trillion. As a result, most of the US government’s spending no longer goes for education, health care, parks, and infrastructure, but to cover the costs of war. It is hardly surprising that many Americans have turned sour on these conflicts.

But if the heavy burden of wars has disillusioned many Americans, why are they so easily suckered into supporting new ones?

A key reason seems to be that that powerful, opinion-molding institutions – the mass communications media, government, political parties, and even education – are controlled, more or less, by what President Eisenhower called "the military-industrial complex." And, at the outset of a conflict, these institutions are usually capable of getting flags waving, bands playing, and crowds cheering for war.

But it is also true that much of the American public is very gullible and, at least initially, quite ready to rally ‘round the flag. Certainly, many Americans are very nationalistic and resonate to super-patriotic appeals. A mainstay of US political rhetoric is the sacrosanct claim that America is "the greatest nation in the world" – a very useful motivator of US military action against other countries. And this heady brew is topped off with considerable reverence for guns and US soldiers. ("Let’s hear the applause for Our Heroes!")

Of course, there is also an important American peace constituency, which has formed long-term peace organizations, including Peace Action, Physicians for Social Responsibility, the Fellowship of Reconciliation, the Women’s International League for Peace and Freedom, and other antiwar groups. This peace constituency, often driven by moral and political ideals, provides the key force behind the opposition to US wars in their early stages. But it is counterbalanced by staunch military enthusiasts, ready to applaud wars to the last surviving American. The shifting force in US public opinion is the large number of people who rally ‘round the flag at the beginning of a war and, then, gradually, become fed up with the conflict.

And so a cyclical process ensues. Benjamin Franklin recognized it as early as the eighteenth century, when he penned a short poem for A Pocket Almanack For the Year 1744:

War begets Poverty,

Poverty Peace;

Peace makes Riches flow,

(Fate ne’er doth cease.)

Riches produce Pride,

Pride is War’s Ground;

War begets Poverty &c.

The World goes round.

There would certainly be less disillusionment, as well as a great savings in lives and resources, if more Americans recognized the terrible costs of war before they rushed to embrace it. But a clearer understanding of war and its consequences will probably be necessary to convince Americans to break out of the cycle in which they seem trapped.

Dr. Lawrence Wittner (http://lawrenceswittner.com) is Professor of History emeritus at SUNY/Albany, and can be contacted at wittner@albany.edu. His latest book is a satirical novel about university corporatization, What’s Going On at UAardvark?