Donald Kirk, in the WSJ (4-28-05):

[Mr. Kirk is an American correspondent who covered the wars in Vietnam, Cambodia and Laos from the arrival of U.S. troops in 1965 until the departure of the last combat unit in 1974. He covered Iraq last year.]

... Those who think final defeat was inevitable and the United States could or should have done nothing more for their South Vietnamese allies are guilty of severe memory loss. They have forgotten the panic of millions of South Vietnamese who could not have imagined the U.S. would desert them after having made one do-or-die commitment after another. By the most conservative estimate, more than two million South Vietnamese fled the country, many by terrifying boat trips through pirate-infested waters, while another 500,000 were sent to "reeducation" camps where untold thousands died either at the hands of their captors or of starvation and disease.

Our betrayal of South Vietnam may have faded into history, a dark moment that Americans can rationalize as a vast mistake from which the United States recovered through later triumphs, notably the overthrow of the regime of Saddam Hussein in Iraq two years ago. But that view would be far too optimistic. That's especially important to keep in mind when some elements in American politics and society seriously contemplate another betrayal -- the notion of pulling out of Iraq, leaving the people we pledged to defend to the mercy of some of the world's most ruthless killers.

Although the United States faces an entirely different enemy in Iraq, the wars bear certain resemblances. As in Vietnam, U.S. forces in Iraq are encountering problems that were never anticipated and have proven far more intractable than military and political analysts expected. As in Vietnam, we are counting on a vast civilian aid program to produce miracles, rebuilding the infrastructure, revamping the financial system, supporting an elected democratic regime.

There is no doubt, moreover, that millions of Iraqis expect the United States somehow to bring about order. If their confidence has diminished after the months of car-bombings, roadside explosions and political assassinations, they're hoping for security and stability of the sort that only Washington can begin to create. No one imagines that the zealots responsible for kidnappings and beheadings would stand on the side of justice and mercy in dealing with those who have cast their lot with the civilian government and bureaucracy, much less the nascent military and police forces.

The danger is that U.S. resolve will weaken in Iraq as it did in Vietnam. After several months in Baghdad last summer and fall, I can say with certainty that it's far more dangerous getting around the Iraqi capital than it ever was in Saigon except during offensives that were always of limited duration. And no one considers venturing alone in the Iraqi countryside, as was often possible in Vietnam. The war in Iraq may in a sense be smaller scale than that in Vietnam, to which Washington at one stage committed more than half a million troops, but it is every bit as difficult, and its outcome is likely to affect if anything more lives when one considers the implications for the region in the context of the global war on terrorism.

It is tempting, 30 years after "the end" in Vietnam, to contemplate where and why the United States failed so badly. That war, as everyone agrees, represented vast miscalculations, but historians will argue on exactly where "the" mistake was made. Certain truths, however, are clear.

The United States could not have "won" without invading North Vietnam on the ground and without sending troops into neighboring Cambodia and Laos to stay. The concept of a "limited war" -- limited in every sense, by Richard Nixon's 60-day time frame for the Cambodian campaign of 1970, by bombing below and above this and that parallel, by bomb halts, by pauses for negotiations, by geographical boundaries -- was ridiculous. U.S. forces, if they were to have had a chance of winning, had to be free to operate unfettered by rules made up by policy-makers in Washington who knew nothing of the daily realities confronting troops on the ground.....

There is a danger the United States will fall into similar rationalizations for pulling out of Iraq. Senior officials in Washington would have us believe the Iraqi armed forces can "replace" American troops in Iraq in a circumscribed period. The point, though, is the United States' commitment, as far as most Iraqis are concerned, is to stay on as long as it takes. It would be a betrayal of trust to attempt to stick to an artificially contrived schedule designed to appease domestic critics and political foes. Similarly, Washington should set no artificial rules of war that hobble U.S. troops in pursuit of the enemy.

If there were any "lessons" to be learned from Vietnam, they were these: We cannot betray our friends or allies who look to us for survival, and we cannot betray our own forces by sending them into war without full commitment to victory. We made those mistakes in Vietnam. We cannot make them again.