Editor's note: Andrew Wiest is professor of history at the University of Southern Mississippi, where he is co-director of the Center for the Study of War and Society. He is the author of "Vietnam's Forgotten Army: Heroism and Betrayal in the ARVN," (New York University Press, 2008).

Hattiesburg, Mississippi (CNN) -- After the fall of South Vietnam in 1975, U.S. Col. Harry Summers remarked to his North Vietnamese counterpart, "You know you never defeated us on the battlefield." After a moment, the North Vietnamese officer replied: "That may be so, but it is also irrelevant."

Although that blunt exchange took place nearly 35 years ago, it's still worthy of close consideration in light of America's wars in Iraq and Afghanistan.

Americans did win their battles in Vietnam, but, as the outcome of the war made clear, raw battlefield prowess did not lead to victory. Why? Because the war there was not for Americans to win or lose. It was a Vietnamese war.

Only South Vietnam had the capability to transform battlefield success into strategic sustainability. In the end, South Vietnam had to be both capable and worthy of its own survival.

But, after the U.S. withdrew the bulk of its combat forces from Vietnam in 1973, after more than 20 years of effort and eight years of war, South Vietnam only held out until April 1975. What went wrong? Why was so much sacrifice in vain? And what lessons might history offer to make our efforts in Iraq and Afghanistan less transitory?

First, we must examine the old chestnut regarding the American understanding of the Vietnam War; the idea that we simply backed the wrong Vietnamese. The idea that, no matter what, the South was never viable. South Vietnam certainly had its share of problems -- endemic corruption, endless political infighting, and a sense of nationalism so weak it couldn't hold back a massive uprising. For all of its failings, however, South Vietnam fought long and hard against difficult odds.

In South Vietnam, during 20 years of war, more than 200,000 people were killed in battle, as many as 1 million civilians died and 1.5 million fled the fallen country as refugees. This is not the story of a nation that did not fight and was simply doomed to defeat. Indeed, South Vietnam's government and its military, though troubled, were arguably much more functional than those of the regimes in Iraq and Afghanistan today.

The real story of the Vietnam War lies in the flawed nature of the U.S.-South Vietnamese alliance. Our raw power combined with their will should have produced victory.

The failures of the alliance were legion, ranging from a lack of reform on the part of the South Vietnamese to an overarching hubris on the part of the Americans. The most significant failing, though, was perhaps the simplest. In the end, the South Vietnamese military was not Vietnamese enough to survive our eventual departure from the war.

With the very best of intentions, American advisers in South Vietnam labored to create a military there that looked very much like the American military. Flush with U.S. aid, South Vietnam built a military based on lavish logistical support and the primacy of overwhelming firepower -- a military that was only an adjunct to the American war in Vietnam.

The Army of the Republic of Vietnam was created to fight alongside its American sponsors in crushing battles of annihilation. Battles, when all else failed, that relied on air- and artillery-delivered firepower to save the day. What is often forgotten is that the plan worked remarkably well, as long as American forces and or support was close at hand.

But the South Vietnamese military was never meant to fight on its own. Once American troops had gone back to "the world" and U.S. funding for the continuation of the war in Vietnam dried up, the doom of South Vietnam was clear.

The tattered, war-torn economy of what was, after all, a Third World nation could never hope to supply the needs, in either manpower or firepower, of its First World-style military. The North Vietnamese invasion of 1975 found a South Vietnam littered with parked and unusable aircraft, tanks that quickly ran out of gas, artillery that could only fire a few rounds per day, and a demoralized infantry.

Today, Americans are urging the Afghans to fight alongside a major U.S. advance in the Helmand Province. With the aid of capable U.S. advisers, and as the recipients of overwhelming U.S. logistical and firepower support, the Afghans will, no doubt, achieve some notable success.

But one day, possibly soon, American forces will withdraw, and -- good intentions not withstanding -- American funding for continued operations in Afghanistan will dry up. We must ask ourselves, is the state and military we are helping to create in Afghanistan "Afghan" enough to survive very long after our withdrawal?

The opinions expressed in this commentary are solely those of Andrew Wiest.