Even now, American troop surge that has transformed the landscape of possibility in Iraq, rarely a day goes by without some public figure pronouncing Iraq “another Vietnam” — adding one more name to a long list that includes the likes of Harry Reid, Muqtada al-Sadr, Michael Moore, Ted Kennedy, Chuck Hagel, and bin Laden deputy Ayman al-Zawahiri. In times of war, it is common for politicians and pundits to brandish and bungle historical analogies. Historians, however, are supposed to stop the false analogies in their tracks, although it has become more difficult because academic historians have largely abandoned military history. Exposing bad analogies is the purpose of “Iraq and the Lessons of Vietnam” (New Press, 336 pages, $25.95), a compendium of essays by such prominent historians as Andrew Bacevich, Gabriel Kolko, Walter Lafeber, Alfred McCoy, John Prados, and Marilyn Young, most of whom know war better than their professorial colleagues. Unfortunately, most of the book relies on the orthodox anti-war narrative of the Vietnam War, which is deeply flawed, and on the anti-war narrative of the Iraq War, much of which is inaccurate, misleading, or unproven. Except for portions of a few chapters, particularly those by Mr. Bacevich, David Elliott, and Lloyd Gardner, the book draws mistaken comparisons and offers lessons that will do more harm than good in Iraq.

Vietnam and Iraq present some obvious similarities and differences upon which everyone can agree. Both are protracted wars with mounting American casualties and declining American public approval. Both feature insurgencies with significant local support. The centrally directed rural insurgency of Vietnam contrasts distinctly with Iraq’s multiple fragmented urban insurgencies. But most of the critical points of comparison are in dispute.

First among these is whether the merits of American intervention in Vietnam in 1965 are comparable to the merits of the Iraq invasion in 2003. The consensus answer in this book is in the affirmative — Iraq is like Vietnam because in neither war did America face a threat serious enough to justify war. These historians summarily dismiss the domino theory of the 1960s, which provided the basis for that war. Recent revelations, however, have resuscitated the domino theory. During the first months of 1965, most Asian leaders believed that South Vietnam’s defeat would cause Asia’s countries to turn communist or pro-communist and make them “little brothers” of China, and for that reason Asia’s non-communist leaders favored American intervention in Vietnam. Japan, Asia’s mightiest country, was in serious jeopardy of breaking away from America if Vietnam fell. Johnson’s commitment of American ground troops to the war in mid-1965 saved a critical domino almost immediately, by inducing the Indonesian military to overthrow the pro-communist Sukarno and destroy the Indonesian Communist Party.

Based on the limited amount of information currently available, the reasons for war in Iraq in 2003 seem less compelling than the reasons for war in Vietnam in 1965. The book’s contributors are correct in noting that the American government misunderstood the stakes in 2003, based upon inaccurate WMD intelligence and rosy predictions about post-Saddam Iraq. But no firm judgment about the merits of the war can be made until the war’s outcome becomes clear and the hidden facts emerge. Much of the aforementioned information on the domino theory became public only last year, 31 years after the war ended.

It is also too early to liken prewar intelligence on Iraq to the Tonkin Gulf incidents of 1964, which several of the book’s authors do. We know for certain that President Johnson willfully misled Congress about events leading to the Tonkin Gulf incidents, and that he falsely assured Congress that the Tonkin Gulf Resolution was less than a declaration of war. The limited evidence currently known to the public suggests that President Bush’s inaccurate announcements about WMD resulted from his belief in flawed intelligence, rather than from deliberate misrepresentation.

American senators, Iraqi militia chieftains, left-wing polemicists, and Al Qaeda leaders invoke the Vietnam-Iraq analogy, above all, as evidence that America cannot win in Iraq. The book’s contributors allege that American and South Vietnamese counterinsurgency initiatives consistently failed to damage the Viet Cong insurgents or deprive them of popular support. Yet overwhelming evidence, much of it from Vietnamese communist sources, shows that the counterinsurgents seriously weakened the Viet Cong in 1962 and 1963 and destroyed them between 1968 and 1971. During the last years of the war, the communists fought a strictly conventional war, using troops imported from North Vietnam.

Of the authors who discuss the efficacy of counterinsurgency in the two wars, only Mr. Elliott pays much attention to Iraq itself. Mr. Elliott disputes the contention of counterinsurgency theorists that America’s woes in Iraq resulted from failure to employ proper counterinsurgency techniques and transfer authority quickly to Iraqis. Mr. Elliott rightly points out that this argument disregards the Iraqi leadership problems stemming from de-Baathification, the dissolution of the army, and social fragmentation, which were worse than anything encountered in Vietnam.

Leadership within the Iraqi government remains a disturbing problem. Events in both Iraq and Vietnam, though, show that leadership can improve unexpectedly, and sometimes quickly and dramatically. In Vietnam, the quality of South Vietnam’s local leaders soared in 1962 as a new generation of officers came of age. After dipping back down in the mid-1960s because of political turmoil, the caliber of South Vietnamese leaders rebounded in the late 1960s. In Iraq’s Anbar province, widely deemed hopeless a year ago, the war has turned around recently because strong Iraqi leaders have joined the American side. The book’s authors, and countless others, point to the ultimate defeat of South Vietnam in 1975 as proof of South Vietnamese ineptitude. In actuality, the South Vietnamese lost not because they lacked skill or popular support but because they lacked the supplies to keep fighting a conventional war. During the Easter Offensive of 1972, after all American ground forces had departed from Vietnam, South Vietnamese forces defeated a massive conventional onslaught by 14 North Vietnamese divisions, and they likely would have done the same in 1975 had Congress not slashed American military assistance, leaving the South Vietnamese without fuel for their aircraft or ammunition for their artillery, and without the American air support Richard Nixon had promised in 1973.

The aspects of the Vietnam-Iraq analogy absent from this book, and indeed from most public discourse, are as important as those present. The book does not mention that in both Vietnam and Iraq, America fought murderous fanatics while supporting indigenous people of more humane and moderate character. In both wars, the irresolution of the American government and the anti-war pronouncements of American citizens and congressmen emboldened the enemy. Ho Chi Minh began sending North Vietnamese Army divisions into South Vietnam in November 1964 because Lyndon Johnson’s inaction and American public rhetoric convinced him that America would not fight for Vietnam. With Iraq, congressional denunciations of the war and threats to cut off funding have certainly encouraged the insurgents. Presidential vacillation would encourage them exponentially.

In both Vietnam and Iraq, America hurt itself by trying to transplant liberal government to a country that, because of cultural differences and the presence of civil warfare, did not possess the right soil for liberalism. America’s reckless imposition of liberalization on South Vietnam in late 1963 and 1964 enabled the Vietnamese communists to infiltrate the cities, spawned popular protests that caused the Saigon government to lose face, and compelled the government’s leaders to do the bidding of self-serving factions. In Iraq, precipitate liberalization created a divided and weak government, undermined social cohesion, and permitted the growth of hostile insurgent and vigilante organizations.

Congressional Democrats tried to slash funding for both the Vietnam War and the Iraq War. Both times they did it less out of disgust with the war itself than out of disgust with the president. Both times they paid scant attention to the human misery that would likely result from American withdrawal. We know what happened in Vietnam: Congress succeeded in cutting the funding, the South Vietnamese consequently lost the war, and the North Vietnamese executed tens of thousands of people and caused hundreds of thousands of others to perish in re-education camps or escape boats. The human costs of an American withdrawal from Iraq would probably be at least as high.

The strategic costs of abandoning Iraq in 2008 likely would be considerably higher than the strategic costs of abandoning Vietnam in 1975. Protecting South Vietnam was less important strategically in 1975 than in 1965 because American actions in the intervening decade bolstered the dominoes and undermined the communists who were threatening to topple them. By contrast, occupying Iraq is more important now for America than it was at the beginning, for an American pullout today would result in violence and chaos that could well spark regional conflict, abet international terrorists, and compel future American military intervention. As Americans decide whether to make Iraq more like Vietnam by bailing out, therefore, we must focus not on Vietnam in 1975, nor on Iraq in 2003, but on Iraq in 2008.

Mr. Moyar, an author of two books on the Vietnam War, holds the Kim T. Adamson Chair at the U.S. Marine Corps University.