A new thesis about the end of the Vietnam war is making the rounds in the context of the debate over Iraq. It holds that President Nixon and Henry Kissinger — not the Democratic Congress and public opinion — were chiefly culpable in America’s betrayal of South Vietnam.

The managing editor of Foreign Affairs, Gideon Rose, is the most vocal proponent of this revision of history. According to Mr. Rose’s writing in Slate, “the settlement the Nixon administration negotiated left the South vulnerable to future attacks.” More recently, writing for the New Republic online, Rick Perlstein stated, “there is a popular fantasy that liberals in Congress, somehow, at least metaphorically, abandoned American troops in Vietnam.”

The importance of this argument has to do with the debate that is taking place for the 2008 presidential election. There is a growing sense that the Democratic leadership in the Congress will try to force a retreat in Iraq by defunding the war, which is what happened in Vietnam.

In the spring of 1975, Congress denied to President Ford’s administration the funds to provide military aid to Vietnam, whose army was forced to retreat from the Central Highlands. The communist conquest quickly followed. Democratic Party intellectuals are working to deflect an argument that the party is trying to do again what it did in the mid-1970s.

Mr. Perlstein, writing in the New Republic, suggests that the votes of Congress did not directly imperil American soldiers, who had already been withdrawn. But that is pettifogging. Both he and Mr. Rose fall off-target in suggesting that Capitol Hill did not abandon the cause for which the GIs fought — our allies in South Vietnam.

A contrast of two military offensives conducted by the People’s Army of North Vietnam highlights their error. In the first offensive, in March 1972, North Vietnam hurled 14 conventional divisions, including 1,200 tanks, into South Vietnam. Nixon authorized American B-52 Stratofortresses into action to help the South Vietnamese army, the primary ground force in Vietnam at the time, fend off the invasion. The enemy sustained more than 100,000 casualties. The offensive failed. In the second offensive, three years later, North Vietnam launched the Ho Chi Minh campaign. Columns of enemy armor, unimpeded by American airpower, sped south, ultimately taking Saigon. At the end of the war, enemy missiles were pulled by tractor-trailer trucks out of the jungle, just miles from Saigon. Messrs. Rose and Perlstein fail to account for how these two similar campaigns ended with tragically different results.

Between 1972 and 1975, America’s Congress passed a series of pieces of legislation that strangled the Republic of South Vietnam of resources and blocked any hope of an American air campaign. While Mr. Rose himself acknowledges that “in June 1973, Congress ordered all American military operations in Indochina to cease by the end of the summer, and in November it passed the War Powers Act,” he soft-peddles the ramifications of these moves — as well as neglecting other legislative restrictions on helping South Vietnam.

These included the Second Supplemental Appropriations Act for Fiscal Year 1973, which blocked funding to “support directly or indirectly combat activities in or over Cambodia, Laos, North Vietnam or South Vietnam”; the Continuing Appropriations Act for Fiscal Year 1974, and the Foreign Assistance Act of 1973, which went so far as to prevent third-party countries from assisting the South Vietnamese so long as they received American aid.

The secretary of state at the time, Henry Kissinger, in his memoir, “Years of Upheaval,” cites those legislative measures as imperiling not only South Vietnam but also Cambodia, where the Khmer Rouge, backed by Communist China, implemented a genocide.

The managing editor of Foreign Affairs makes much of Mr. Kissinger declaring “peace is at hand,” but he neglects to tell the rest of the story. The authors of the biography “Kissinger,” Marvin and Bernard Kalb, recount that Mr. Kissinger raised a host of concerns at the same briefing famous for the “peace” statement. He warned of seven other issues, “including the problem of discouraging a land-grabbing operation by the North Vietnamese.” The Kalbs conclude: “Lost in the exultation was Kissinger’s mention of ‘nuances,’ ‘differences’ or ‘six or seven concrete issues.'”

Mark Moyar, author of “Triumph Forsaken: The Vietnam War, 1954 –1965,” is part of a new wave of historians challenging the leftist version. Now at work on a book that tells the story of the second half of the war, Mr. Moyar insists that an Easter Offensive-style bombing campaign would have at least delayed the defeat of South Vietnam for a significant period of time and perhaps even put the North Vietnamese on their heels for a while.

“In the offensive in 1975, the North Vietnamese are moving around huge conventional forces that would have been pulverized by our air power,” Mr. Moyar told The New York Sun this week. From his study of the official North Vietnamese history of the war, he learned that the conquering forces were well aware of the effect of the congressional actions. “In 1974 fire support by tactical aircraft and artillery decreased 75 percent in comparison with 1972,” he cited the history as saying.

Moreover, Mr. Moyar said the North Vietnamese only attempted their 1975 attack when convinced that America would not counter this violation of the Paris Agreement. He supplies the North Vietnamese Official History. The conquest of Phuoc Long in January 1975 served as “a kind of ‘strategic reconnaissance’ for us. … The victory exposed the limited ability of the United States to react after the forced withdrawal of their expeditionary army from South Vietnam.”

Mr. Kissinger had become aware of the North Vietnamese close-reading of American politics when he obtained an enemy document containing North Vietnamese instructions to their Viet Cong cadres in May of 1973. According to Mr. Kissinger’s book, the document stated that Nixon’s “weakened authority over the U.S. government is now generating a favorable influence in South Vietnam for the struggle of the NLF, and will result in a new U.S. policy in Indochina. Even if President Nixon remains in office … he will not dare to apply strong measures as air strikes or bombing attacks in either North or South Vietnam, because the U.S. Congress and the American people will violently object.”

Nowhere in Mr. Rose’s essay is there an acknowledgement of the North Vietnamese response to congressional actions. Both the document obtained by Mr. Kissinger and the Official History, via Mr. Moyar, make clear that the enemy profited by congressional action, and knew it.

During the final days of the South Vietnamese government, supplies and equipment were in short supply. Speaking at the John F. Kennedy Library’s conference on “Vietnam and the Presidency” last March, Mr. Kissinger said “when you cut military assistance from $2.4 billion to $700 million, when you prohibit military action in the face of the most blatant violations, you are bound to lose.” Mr. Moyar speaks of former Vietnamese commanders who told him about their being limited to firing one artillery shell a day.

There is a word for that, and that is betrayal. Without a doubt, Congress felt compelled to follow the public and leave South Vietnam defenseless in 1975. But with the hindsight of history, we know that measures existed that could have preserved the South Vietnamese government without full-scale American redeployment, namely the air war, money, and supplies. Attempts to absolve the 93rd and 94th Congresses and to shift the blame for the final fall of Saigon to Messrs Nixon and Kissinger only cloud today’s current leadership from acting wisely as we are challenged by a new and equally savage enemy.

Mr. Gitell (gitell.com) is a contributing editor of The New York Sun.