“Maybe one hundred,” Mr. McCain replied. “As long as Americans are not being injured or harmed or wounded or killed, it’s fine with me and I hope it would be fine with you if we maintain a presence in a very volatile part of the world where al Qaeda is training, recruiting, equipping, and motivating people every single day.”

All hell broke loose in the media. Democratic candidates then-Sens. Hillary Clinton and Barack Obama swiftly condemned what was widely regarded as a gaffe. On CNN Mr. McCain felt obliged to clarify. “It’s not a matter of how long we’re in Iraq,” he explained, alluding to our military presence in Japan, Germany and South Korea, “it’s if we succeed or not.”

 

Sen. John McCain (R., Az.) while speaking at a town hall meeting in Salem, N.H., in 2008. Getty Images

In retrospect, Mr. McCain was correct: He predicted that setting a timetable for withdrawal would mean “chaos, that means genocide, that means undoing all the success we’ve achieved.” This is now happening. But the press consensus, then as now, was that the American people were war-weary and a President McCain would perpetuate Mr. Bush’s “failed” policies.

The truth is that at any given time the American people are weary (or wary) of war. They were surely war-weary in 1864, when Abraham Lincoln did not expect to be re-elected; and they were war-weary in early 1942 when Pearl Harbor had been bombed, the U.S. Navy devastated and the Army had surrendered to the Japanese on the Philippine peninsula of Bataan. In 1949, there was a precursor of sorts to the McCain controversy when Secretary of State Dean Acheson was asked by an inquisitive senator if American troops might have to be stationed in postwar Western Europe to fortify NATO, and whether they might remain for five, or perhaps eight or 10, years?

“The answer to that question,” replied Acheson, “is a clear and absolute ‘no.’ ”

Acheson immediately regretted his response, and American troops were dispatched to Europe, where they remain 65 years later. U.S. troops still patrol the border between South and North Korea as well. This commitment has been expensive and perhaps wearisome for the soldiers involved. But would anyone suggest that our postwar commitment to European security was ill-advised, or that South Korea should be left to the mercies of the North? Very few shots have been fired in these outposts, but they have kept the peace and transformed the continents of Europe and Asia.

We have here an object lesson in statecraft: The duty of political leaders in perilous times is to lead, not follow, public opinion. And “war-weariness,” whatever that means, is no excuse for dereliction of historic duty.

The much-admired former Senate Majority Leader Mike Mansfield (D., Mont.) routinely demanded the withdrawal of American troops from NATO, and just as routinely, senators from both parties rejected his counsel. But in the same year (1971, at the height of the Vietnam War) that the Mansfield Amendment came closest to success, gaining 36 votes, Democratic Sen. George McGovern began his presidential campaign with its theme of “Come home, America.” McGovern was clobbered in the election, but 42 years later his isolationism is mainstream doctrine in his party.

The inconvenient lesson is that the withdrawal of American peacekeeping forces, in modern history, has almost invariably led to catastrophe. The departure of federal troops from the Reconstruction South paved the way for Jim Crow. The swift American exit from Europe after 1918 subtracted from the continent’s collective security, leading to World War II. It might even be argued that the stabilizing influence of American troops in Haiti, Nicaragua and even interwar China did considerably more good than harm.

It is impossible to predict Iraq’s future, and equally impossible to anticipate events in neighboring Syria. The sobering fact is that President Obama’s disengagement from the region is a matter of political principle and he is unlikely to alter his course. But the evidence of American withdrawal is now palpable, and as clear as the images of ISIS atrocities. So which is more wearisome: The resolve to sustain our burden of leadership, or the prospect of a world in chaos and uncertainty?

Mr. Terzian, literary editor of the Weekly Standard, is the author of “Architects of Power: Roosevelt, Eisenhower, and the American Century,” (ReadHowYouWant