When Republicans were attacking Democratic presidential candidate John Kerry in 2004 over the supposed exaggeration of his Vietnam record, an unexpected ally came to his aid: John McCain. The Arizona senator and current Republican presidential nominee, who spent five-and-a-half years in a North Vietnamese prison camp during that war, chastised those who sported purple Band-Aids at the GOP convention to mock Sen. Kerry’s Purple Heart medals.

That year, Sen. McCain told USA Today: “I’m sick and tired of re-fighting the Vietnam War. And most importantly, I’m sick and tired of opening the wounds of the Vietnam War, which I’ve spent the last 30 years trying to heal. It’s offensive to me, and it’s angering to me that we’re doing this. It’s time to move on.”

That sort of response – surprising, sharp and noble – is why many Americans, regardless of political stripe, respect and admire Sen. McCain. It is indeed time to move on. But the shadow of Vietnam will likely fall over the coming campaign, despite Sen. McCain and many others wishing otherwise. When it does, the candidates’ views of that war could clearly delineate their foreign policy positions.

Here is the key question for the candidates: Was the U.S. involvement in Vietnam a mistake? Why or why not? That question is in some ways easier to ask than the underlying question, which is whether invading Iraq was a mistake. The candidate’s answers could reveal how they would conduct foreign policy.

With Vietnam, we have the benefit of 30-plus years of hindsight. Policymakers of the time, we now know, believed the economic and political ideology of Communism was being spread at gun point around the world. The U.S. had to draw the line against such incursions or risk a much diminished sphere of influence. The phrase “We’ve got to fight them there or we’ll be fighting them here” remarkably was used in defending the Vietnam policy, just as it is now used by the Bush administration on Iraq policy.

Perhaps the biggest lesson that should have come out of the Vietnam debacle is that it is impossible to fight an idea with guns. North Vietnamese leader Ho Chi Minh coupled communism with nationalism in a package that was irresistible to residents of a country that had been colonized and occupied by outsiders for centuries. The U.S. misread the situation as the Soviets manipulating the Viet Cong to expand its realm.

The Iraq War is similar to Vietnam, but the motivation was a sort of reverse domino theory with the Bush administration hoping to tip Persian Gulf countries toward democracy and away from theocracy. The idea under assault this time is radical Islam. Like communism, it cannot be defeated with guns.

Unprovoked military action and occupation by the U.S. – as was the case in Vietnam and Iraq – has rarely yielded good results. Should Sen. McCain or Sen. Obama not understand the lesson of Vietnam, they will be prone to drag the U.S. into another occupation doomed to failure.