President Barack Obama and his aides were surprised this month by the strength of public opposition to their call for military action against Syria. They shouldn't have been.

Americans have almost always been reluctant to go to war. In 1939, polls showed most Americans not only wanted to stay out of war against Nazi Germany, they weren't even sure they wanted to send military aid to Britain, perhaps fearing a slippery slope.

Today, Americans have additional reasons to be skeptical. There's the toll of wars in Iraq and Afghanistan. There's the fear that any war in the Middle East will inevitably become a quagmire. And there's also a fundamental change in American attitudes toward their leaders.

The traditional center in American foreign policy - the rally-around-the-flag reflex - has eroded. One reason is partisan polarization: Many conservatives who might have supported military action under a Republican president are disinclined to help Obama. But it's not all partisan; public confidence in the government's ability to do anything right has reached an all-time low, according to a recent Gallup Poll.

Are Americans becoming isolationists? That's not so clear.

It's true that public skepticism about U.S. engagement overseas is up. The Pew Research Center reported recently that 46 percent of Americans endorsed the sentiment that "the United States should mind its own business internationally and let other countries get along the best they can on their own."

But that isn't unprecedented; Pew found anti-interventionist sentiment almost as high in 1974, at the end of the Vietnam War, and in 1992, at the end of the Cold War. That isolationism didn't last forever.

Americans recoiled from the proposal to attack Syria not only because they are skeptical about military adventures but because they weren't convinced that this venture was in the national interest.

"This was kind of a worst case," said Pew Center Director Andrew Kohut. "The public is very gun-shy about intervention, but especially in the Middle East, and especially in a case where the direct U.S. interest isn't clear. If there were a direct and major threat to the United States, you'd probably see a different picture."

Polls taken before earlier conflicts have shown Americans are willing to support military action when they are convinced that U.S. security is directly threatened. On the flip side, most Americans will not support military intervention for purely humanitarian reasons - as Bill Clinton learned in Somalia, Bosnia and Kosovo, operations that were all widely unpopular at the time.

That's a problem Obama hasn't solved when it comes to Syria. He asked Americans to watch videotapes of children choking on sarin gas in a Damascus suburb - but that was a humanitarian appeal, not an invocation of national security. He argued that Americans had an interest in bolstering international norms against chemical weapons - but that sounded like an abstract principle, not an immediate threat.

Opposition in Congress to a presidential request to use force has a long history. James Madison ran into a roadblock on Capitol Hill in 1815, and Woodrow Wilson lost a vote in 1917. In 1999, the House of Representatives refused to back Clinton's air war over Kosovo; Clinton went ahead anyway after winning a vote in the Senate. And even more recently, in June 2011, the House voted against authorizing Obama's intervention in Libya - one justified mostly on humanitarian grounds - by a lopsided 295 to 123. (The vote came after U.S. strikes on Libya were already under way, and Obama ignored it.)

Americans are often tempted toward disengagement from the world, especially at the end of a long and costly war (in this case, two wars), and especially when the question involves military action. It happened after Vietnam, it happened after the Cold War, and it's happening again today.

But after those earlier episodes, public opinion bounced back. Presidents Ronald Reagan, George H.W. Bush and Clinton made the case for American intervention abroad, and in cases when intervention succeeded, public support grew.

With Syria, it became clear that Obama's request for authority to intervene would be rebuffed. One result is that Americans look and sound more isolationist than they really are. That heightens a challenge that Obama and his successors already faced: not only dealing with a crisis in Syria but rebuilding a national consensus in favor of engagement with the world.

Doyle McManus is a columnist for The Los Angeles Times. Email him at doyle.mcmanus@latimes.com.