From the moment the last U.S. helicopter departed the Saigon embassy in April 1975, the fall of South Vietnam has figured prominently in American debates over the Vietnam War. Doves attribute South Vietnam's defeat to governmental weakness and illegitimacy—reinforcing their argument that America's ally had always been unworthy of support. Hawks blame the country's loss on a cut-off in U.S. aid—a view in line with the hawks' belief that American politicians foolishly squandered what nearly 60,000 U.S. soldiers had died to preserve.
Yet unlike the war's earlier episodes, the final act has received only cursory treatment from historians. Authors of supposedly comprehensive histories of the war tend to speed through the period 1973-75. During that time, Vietnam was devoid of U.S. combat forces and was of scant interest to American journalists and civilian officials; scholars thus had few English-language sources to consult.
George J. Veith's "Black April" fills the gaping historical void, and in extraordinary fashion. Mr. Veith has tapped deeply into previously neglected Vietnamese sources, including North Vietnamese histories, and he has interviewed commanders of numerous South Vietnamese units. In a blow-by-blow account, he presents mountains of new details that enable him to answer the principal historical questions.
Although South Vietnam's leaders committed some critical errors during the North's 1975 offensive, Mr. Veith says, the defeat cannot be attributed to governmental ineptitude—and certainly not to war-fighting incompetence. By then the South Vietnamese military leadership included many officers who had performed exceedingly well in repulsing the 1972 Easter Offensive and in large but little-known clashes of 1973 and 1974. During the final North Vietnamese offensive, South Vietnamese commanders and their units fought much better than has been believed.
If Americans recall a South Vietnamese victory in 1975, it is likely the battle of Xuan Loc, where one division of the South Vietnamese army bludgeoned three North Vietnamese divisions. But Mr. Veith chronicles several other clashes, in March and April 1975, that show the South's stout resistance—from Mo Tau and Bong Mountain in the country's northern extremities, to Ben Cau and Chonh Thanh in the central region, to Can Tho and Long An in the south.
Mr. Veith demonstrates persuasively that the root cause of South Vietnam's defeat was the slashing of assistance by the U.S. Congress in 1974, when military aid was nearly halved. As the North Vietnamese onslaught began in March 1975, South Vietnam's shortages of aircraft fuel and spare parts prevented the military from flying troops in to fortify a vulnerable 900-mile western flank. The North Vietnamese were thus free to focus their attacks with overwhelming numbers on key towns and cities.
Because of the scarcity of air assets, imperiled South Vietnamese troops frequently had to retreat by truck or on foot. Civilians raced after the soldiers, terrified of being massacred by the communist forces, who had slaughtered noncombatants in Hue in 1968 and along Route 1 in 1972. Women and children and civilian vehicles clogged the major roads and bridges, slowing the withdrawal. Consequently, some of the combat units were cut off by the advancing North Vietnamese and destroyed.
By George Veith
Encounter, 587 pages, $29.95
When retreating South Vietnamese forces attempted to form a defensive perimeter at the coastal city of Da Nang, more than a million frenzied civilians flocked to the city, where the 500,000 inhabitants were already in a state of panic. The city streets were so packed with civilian traffic, Mr. Veith explains, that the movement of military vehicles and large troop formations could not be coordinated. Some soldiers left their units to protect relatives or help them flee. Gen. Ngo Quang Truong, a brilliant and charismatic South Vietnamese commander, decided that an organized defense was impossible; he ordered an evacuation of combat troops by sea. Some South Vietnamese soldiers escaped on ships, but many thousands of others were captured on the beaches by onrushing North Vietnamese units.
"Black April" shows that the sharp reduction of U.S. aid meant the South Vietnamese were unable to bomb enemy ground forces even when they were massed and presented inviting targets. The South Vietnamese air force could not fly enough sorties to begin with, and its capabilities further eroded as the North Vietnamese overran airfields. In January 1973, President Nixon had promised the South Vietnamese that American air power would smash the North Vietnamese if they broke the peace agreement about to be signed in Paris. But then Watergate intervened, and Congress—using the War Powers Resolution of 1973—prevented Nixon's successor, Gerald Ford, from fulfilling the pledge.
Although South Vietnam had 763,000 men under arms in 1975, the military's very limited strategic mobility permitted it to assemble only 110,000 in Saigon for a last stand. The North Vietnamese, brought 350,000 troops to bear and they were richly supplied thanks to the recent transformations of the Ho Chi Minh Trail into a paved highway and the construction of an oil pipeline alongside it; Despite the hopelessness of the situation, Mr. Veith notes, many South Vietnamese units fought off large North Vietnamese assaults on Saigon and launched effective counterattacks. According to Hanoi's own estimates, North Vietnamese forces sustained 6,000 casualties in the war's last days. South Vietnamese soldiers fought on until a newly installed caretaker government ordered a surrender in the vain hope of gaining concessions from the conquerors.
More than 100,000 South Vietnamese who had sided with the United States perished in the final battles, were executed immediately thereafter or died from maltreatment in massive "re-education" camps. Half a million more South Vietnamese died while attempting to flee communist oppression by boat. As the U.S. contemplates its future assistance to Afghanistan, "Black April" provides a sobering reminder of the human costs of abandoning a beleaguered ally.
—Mr. Moyar is a defense consultant and the author of "Triumph Forsaken: The Vietnam War, 1954-1965."
A version of this article appeared May 5, 2012, on page C8 in some U.S. editions of The Wall Street Journal, with the headline: Abandoning Vietnam.