"Example is the school of mankind," proclaimed Edmund Burke, the founder of modern conservatism, "and they will learn at no other."

Burke was disparaging the folly of French revolutionaries who believed that man could break the iron chains of history and create utopias through willpower and planning.

This argument about whether history has anything to teach us has been the essence of the left-right debate for most of the last two centuries. Conservatives said: "There's nothing new under the sun." The left said: "Until now!"

Karl Marx - with a lot of dialectical mumbo jumbo - was the most famous champion of the need to change history, not interpret it. But my favorite summary of this mind-set comes from Stuart Chase, the intellectual often credited with coining the phrase "New Deal" for FDR. "Are our plans wrong?" he asked. "Who knows? Can we tell from reading history? Hardly."

Now, this right-left divide is falling apart, as both sides search for a guiding historical analogy for our current predicament.

The Bush administration is determined to convince the public that it is 1938 in Iran and Mahmoud Ahmadinejad is Hitler. Or that it's 1917 and Osama bin Laden is a new Lenin. Others see Spanish Civil Wars in Iraq or on Lebanon's southern border. I shudder to count all the folks who claim that Iraq is Vietnam.

For many liberals of a certain generation, Vietnam is a universal peg, fitting perfectly into analytical holes of any shape. Indeed, the closest thing we get to a neat left-right divide on foreign policy these days is between those who see Vietnam as the Rosetta stone of international conundrums and those who see early 20th-century Europe as the universal translator.

These analogies have some persuasive power. I am particularly convinced the White House is correct that ignoring the evil of jihadism - whatever its historical analogues - will be repaid with tragedy.

Nonetheless, there are two problems with historical cherry-picking. The first is our collective ignorance about history. As a culture, we have a tendency to look for our car keys where the light is good. Our usable past is the past that is illuminated to us. One reason we leap to analogies about World War II and the Cold War is that it's the only history most of us know. It's telling that military histories about World War II (the "good war") vastly outnumber all others. People don't call the History Channel the Hitler network for nothing. Meanwhile, Vietnam (the "bad war") feels like only yesterday to baby boomer liberals, so they have a tendency to see LBJ, Robert McNamara or other ghosts of "quagmires" past haunting the Bush White House.

Proponents of each example have a specific response in mind. If Iraq is Vietnam, we allegedly have no choice but to give up in Iraq. If the war on terror is like the Nazi menace, it's time to wax Churchillian.

But what if there are historical parallels lurking in the shadows of our ignorance? What if the jihadists are more like the Muslim Barbary pirates made famous in the Marine Hymn with the line about "the shores of Tripoli"? Or maybe they're more like the Thugees, an 18th-century murder cult in colonial India. Or the Panslavist Black Hand. Or a radical faction of the youth group Up With People. OK, I'm kidding about that last one. The point is, we don't know. But surely the ocean of historical experience cannot be summed up by the tributaries of Vietnam and Nazi Germany.

The second problem is that although Burke was right that human nature has no history, technology does. Newsweek's Fareed Zakaria penned a thoughtful essay debunking the 1938 analogy. Iran is an economic and military weakling compared with the Third Reich. Inflating Ahmadinejad into Hitler stretches the analogy to the breaking point. Fair enough, but Ahmadinejad has options Hitler didn't. Der Fuehrer needed a strong economy and an enormous military to accomplish his objectives. Thanks to nuclear and (soon) biological weapons, second-rate powers like Iran, as well as basket cases like North Korea and modern-day Thugees like Bin Laden, can quickly attain destructive power Hitler only dreamed of. As science proceeds, this reality will loom ever more frightening.

Maybe Chase's conviction that history provides few solutions is finally vindicated, as weapons of mass destruction are something new under the sun. It's ironic that just when the left has come to admire the utility of history, history may be offering us a blank page. The sobering question is: What kind of analogy will we provide for future generations?