As the presidential campaign goes on, we will continue to hear a lot of comparisons of Iraq and Vietnam. Especially with John McCain, a true Vietnam hero, as the emerging Republican nominee. The two wars are both different and alike.

Both were mistakenly undertaken. Both have been unpopular at home and abroad. Both have been costly in American casualties and money. Both have caused disastrous destruction and loss of life among native civilians.

Sen. McCain, who predicts eventual “success” and “victory” in Iraq, has noted key differences in the two wars. He has said that, unlike the Vietnamese enemy, the enemies in Iraq had no superpower supporting them. He could have added that Vietnam had no oil.

The biggest difference, of course, is that the Vietnam War ended in an American defeat, while no end is in sight in Iraq. For the United States, the venture in Vietnam ended in the worst possible way, with remaining U.S. soldiers and diplomats, and some allied Vietnamese escaping by helicopter from a rooftop, with a few desperate people clinging to the landing skids. It is true that President Nixon tried unsuccessfully to “Vietnamese” the war and that Congress finally cut off funds. But it was also true that the Vietnam War was unwinnable and that the Vietnamese were fighting for keeps while the American troops mainly wanted to go home when their year was up. That war has left such a sour taste in the American mouth that most politicians still avoid calling it a defeat.

As things have turned out, however, Vietnam has become a valued trading partner and a peaceful and stabilizing force in Southeast Asia. President Bush, in his 2006 visit to Hanoi, hailed Vietnam’s booming capitalistic economy and its peaceful policies while pressing its communist leaders for greater political and religious freedoms.

What if we had won in Vietnam? No one can say for sure, but it seems likely that the United States would still be an occupying force, with tens of thousands of its soldiers hunkered down in permanent bases and undergoing continual sniping and bombing and booby trapping whenever they ventured out. Or else, an American force might be stationed in a neighboring country or offshore in naval vessels, with an occasional helicopter shot down when patrolling the enemy countryside. Guerrilla warfare always has an advantage in numbers and in strategy over an occupying army.

Will the United States be better or worse off when some future president declares victory or calls a halt in Iraq? The long American adventure in Vietnam may suggest answers to present questions about Iraq: How long should we keep it up? How can we keep allied countries on board? And, finally, what would victory or defeat look like?

Presidential candidates must be considering these long-term questions. One of them will soon be in charge.