For those who are worried about whether America is going to stick with Iraq — could this be a marker for other allies? — I like to recommend paying attention to the abandonment of Free Vietnam. I’ve been thinking of it again this week as the world waits to see what, if anything, America is going to do in the face of the advance against Iraq by the forces of the Islamic State of Iraq and Syria.

Vietnam, for many of us, is the defining war of our generation. I covered Vietnam as an enlisted man in the American Army assigned to the GI daily, Pacific Stars and Stripes. I was in Hong Kong for the Wall Street Journal when, in the spring of 1975, Congress voted to cut off support for the South Vietnamese. Like other correspondents, I raced to Saigon. We all knew what was about to happen.

It was like watching history unfold on wide-screen cinema. Congress, overriding a veto by President Gerald Ford and pleadings by Secretary of State Kissinger, voted to end all support for the government of the South Vietnamese Republic. South Vietnam’s doughty president, Nguyen Van Thieu, had little choice but to bring his divisions out of the Central Highlands and fall back toward the capital.

The communists completed their conquest in a matter of days. When they came out of the jungles they weren’t carrying pitchforks and spears, contra the Left, which had it that these were the weapons used to defeat our helicopters and tanks. They emerged in tanks and hauled surface to air missiles that had to be towed by heavy trucks. That’s how the communists completed their conquest, dooming millions to die in re-education camps or perish on the high seas or in the killing fields of Cambodia.

What stays with me over the years is the absence of ruth on the part of the Congress. It was no longer a question of saving our GIs. There were no American combat troops — zero — left in Vietnam by the time Congress betrayed Vietnam. This was an abandonment of an ally, a country for whose right of self-determination America had sacrificed more than 58,000 of its own GIs. And with which America was a signatory of the Paris Accords.

Mark that our original decision to send an expeditionary force had been made by a resolution that passed the Senate in 1964 by a vote of 88 to two and the House by 416 to zero. The resolution was passed after attacks on our vessels in the Tonkin Gulf. One of the attacks has since been exposed as exaggerated or even fictitious, and LBJ used the Tonkin Gulf resolution to take us deeper into Vietnam than intended, which is one reason why the near-unanimity failed to dash the controversy that enveloped the war. When Congress turned against the war, it was sudden and devastating.

This is the danger in Iraq. The 2002 Iraq War Resolution passed by an overwhelming bi-partisan margin, not as astounding as Tonkin Gulf, but still 296 to 133 in the House, and 77 to 23 in the Senate. American armed forces suffered 4,487 combat fatalities, according to figures cited by Wikipedia — that is, fewer than a tenth of the sacrifice in Vietnam. President Obama ordered our last combat GIs out of Iraq in 2011.

So as was the case in Vietnam, the decision President Obama is wrestling with is not about saving American combat fatalities but about saving a Free Iraq. As was the case in Vietnam, there are a lot of patriotic people on both sides of the issue. And it’s not my purpose here to debate whether America should go back in —be it with troops, air power, covert forces, or merely materiel. It is about betting on America.

Kissinger once warned that were American to turn against South Vietnam’s President Thieu, “the word will go out to the nations of the world that it may be dangerous to be America’s enemy, but to be America’s friend is fatal.” The idea has been widely embroidered. It strikes me as the most dangerous feature of the current crisis. It would be a mistake — we learned this in World War II, among other fights, and the Cold War — to underestimate America. But Vietnam counsels caution.

When Vietnam fell, the Jewish Daily Forward — this is long before I edited its English-language edition — issued a famous warning. Of the communists, this tribune of labor anti-Communism said, “a million Vietnamese victims in the long and bloody war was not enough for them.” It warned that now the “bloody revenge of the Communist against their opponents will only truly begin.” That’s the thing to think about as President Obama moves the carrier George H.W. Bush into position and decides whether to go back into combat.

Seth Lipsky is editor of The New York Sun www.nysun.com. He was a foreign editor and a member of the editorial board of The Wall Street Journal, founding editor of The Forward and editor from 1990 to 2000.