The Vietnam war has been widely regarded these past 40 years as a total American defeat and a huge mistake. Yet consider what actually prompted us to become involved with Vietnam in the first place. It was, after all, Vietnam that got us into World War II.

We somewhat tolerated Japan’s rampaging all over China, but when it invaded what is now Vietnam in 1940, we lowered the boom on Japan promoting embargoes on oil, scrap iron, and rubber. This posed a threat to its economy. To eliminate the threat to its future expansion into Southeast Asia, it attacked our fleet at Pearl Harbor.

President Eisenhower no doubt had this in mind when, at a 1954 press conference, he was asked about “on the strategic importance of Indochina for the free world.” He warned of “broader considerations that might follow what you might call the ‘falling domino’ principle.” Ike spoke of the possible loss of Indochina, Burma, Thailand, the [Malay] Peninsula, and Indonesia, as well as a danger Japan Taiwan, the Philippins, Australia, and New Zealand.

“You are talking really about millions and millions of people,” he said. The “Domino Theory,” widely belittled in the US, was taken seriously by leaders in Asia. Marshal Lin Piao of communist Chinastated in September 1965 that the defeat of “U.S. imperialism” inVietnam would show the people of the world “that what the Vietnamese people can do, they can do too.”

In the 1970s, Indonesia’s Suharto and Malik, neither of them great friends of America, told columnist Robert Novak that they had earlier told U.S. officials that our introduction of Marines to Vietnam in March 1965 substantially encouraged Indonesian generals, such as Suharto and Malik, to resist the subsequently nearly successful Chinese-supported Communist coup attempt that would have threatened the Philippines, triggering our 1954 treaty obligations to that country.

Our commitment of combat troops to Vietnam did much to encourage Britain’s successful defense of Malaysia against a Communist invasion launched from Indonesia. Our commitment to defending South Vietnam bought time for these countries to strengthen their ability to resist Communist encroachments. By the end of the war in Vietnam, the North had lost more than 2 million dead and was too weakened to pose a threat to any but its neighbors Laos and Cambodia.

In essence, we got into the war to prevent the toppling of dominos in Southeast Asia, and we effectively succeeded, which was, in reality, a strategic victory. It was so even while we suffered a tactical defeat in Vietnam.

The saddest aspect of the Vietnam War is that we snatched that tactical defeat from the jaws of victory. By 1972, the Vietnamization process had been largely completed. There were no more American ground combat forces in Vietnam, only advisors and forward air controllers. We still provided air naval and logistics support. In 1972, America had about 200 killed in action, as opposed to an average of about 7,000 in previous years.

Hanoi decided to test Vietnamization by launching the 1972 “Easter Offensive” — the largest of the war. It used the equivalent of 23 divisions equipped with hundreds of Soviet supplied tanks, long-range artillery and rockets, surface to air missiles, and other modern weapons. It was the first largely conventional enemy offensive of the war.

South Vietnamese ground forces, army and marines, with massive American air, as well as naval and logistics, support, not only stopped the offensive but launched counter attacks. Hanoi’s forces lost 100,000 killed in action, twice as many slain as we suffered in the entire war. The enemy was being defeated.

Sometime after the communist conquest was completed in 1975, a former top commander in the South, General Tran Van Tra, stated, in the Party organ Nhan Dan, that, in effect, his troops were on the ropes and on the verge of defeat. As the former director of central intelligence, William Colby, certainly no hawk, wrote in his 1983 book “Lost Victory”: By the fall of 1972 “on the ground in South Vietnam the war had been won.”

On the verge of defeat, Hanoi made a clever move, which saved it from defeat: It offered substantial concessions long sought by our side in past negotiations. President Nixon, facing a showdown at the polls with an anti-war Democrat, George McGovern, had Henry Kissinger enter the into talks in Paris with his long time North Vietnamese negotiating partner, Le Duc Tho.

This process of negotiations effectively ended South Vietnamese operations that may well have been a few months short of victory. The signing of the Paris Peace Accords in January 1973 supposedly ended hostilities, but were soon massively violated by Hanoi’s forces.

We then greatly reduced military aid to Vietnam, and on June 4, 1973, a Democratic-dominated Congress passed the Case-Church Amendment, which effectively banned all U.S. military operations in Indochina thus sealing South Vietnam’s doom.

North Vietnam, well supplied by its reliable allies, China and the Soviet Union, and having effectively recovered from its near defeat in 1972, in March 1975, launched the attack that for lack of American support of Free Vietnam won the war.

Mr. Stearman, a retired Foreign Service officer, served on the White House National Security Council staff under four presidents. His memoir, “An American Adventure,” was brought out in 2012 by Naval Institute Press.