The Vietnam War entered my consciousness in the fall of 1964. I was a teenager, the son of an American diplomat, living in Pakistan. I vividly recall the sunny morning when my father, a senior official in the US mission in Karachi, came to give a talk at the international school in that sprawling port city. The students, mostly Americans, sat in folding chairs on the lawn. Not far away, in Southeast Asia, other Americans were starting to fall in combat.

That summer, on Aug 7, Congress had passed the Gulf of Tonkin Resolution, giving President Lyndon Johnson the power to act militarily in Southeast Asia. It was as close as the US ever came to a declaration of war in a conflict that lasted for more than a decade and killed more than 58,000 Americans and an estimated three million Vietnamese.

With a huge, brightly colored map of Southeast Asia displayed behind him, my father explained that the war was being fought to draw the line against Chinese expansionism in Southeast Asia. If Vietnam fell to the communists, then one by one, like falling dominoes, other Asian countries would follow. The largest prize, Indonesia, was already close to communist revolution. I was convinced and became, for a time, a true believer in the grand purpose of the Vietnam War.

The "domino theory", first voiced by President Dwight Eisenhower, was the driving geopolitical logic behind the American war in Vietnam. It persisted even after the communists were brutally crushed in Indonesia in 1965-66, and even after it was evident by the late 1960s that the rest of Southeast Asia was secure.

The only dominoes that did fall were Cambodia and Laos, which along with Vietnam had been part of French Indochina. But the dominoes stopped there and, in the case of Cambodia, eventually flipped back. Three decades after the fall of Saigon to the Vietnamese communists on April 30, 1975, the domino theory seems naive and misguided.

Far from being instruments of Chinese expansion, the Vietnamese became barriers to it, starting in 1978. The Vietnamese invaded Cambodia that year to oust Pol Pot and his murderous Khmer Rouge, who turned out to be fiercely anti-Vietnamese and devout followers of the ideology of Mao Zedong. The Chinese cannot be held directly responsible for the genocide carried out by the Khmer Rouge. But there is little doubt that Pol Pot's Cambodia was an instrument of Chinese geopolitical influence in the region - a brutal means to keep Vietnam in check. Although Cambodia was cut off to the world, Chinese aid flowed in and Chinese advisers moved freely throughout the country.

The Vietnamese invasion was partly self-defense, coming after months of attacks along its borders and a flood of Cambodian refugees into Vietnam. But it was also an attempt to assert Vietnamese control and block Chinese expansionism.

The Chinese clearly understood the import of the move to oust Pol Pot. They launched an attack on Vietnam only a few months later. The defeat of the Chinese army at the hands of the war-toughened Vietnamese stung Beijing. But China kept the Khmer Rouge alive for more than a decade, fueling a prolonged and costly insurgency that ended only with a UN-brokered peace and the formation of a coalition government.

In the early 1980s, when I travelled as a reporter to Vietnam and Cambodia, Vietnamese communist officials spent most of their time denouncing communist China. They took me up to the still tense China-Vietnam border to witness the scars of China's brief invasion. Throughout that decade, Vietnam made undisguised attempts to open the door to its old foe, the US. But the US, for reasons of realpolitik, opted instead to align itself with the Chinese, effectively backing the Khmer Rouge in their struggle against Vietnam.

More than another decade had to pass before the US and Vietnam re-established diplomatic relations in 1995. It is no coincidence that the US began to warm up to Vietnam at a time when America's leaders were once again looking warily at the rise of China as a great power, this time not as an agent of spreading communist revolution but as an economic giant fast becoming the factory for the world.

For some Americans, Vietnam was now an attractive link on China's perimeter, from Japan to India, helping to keep Beijing in check. But it may have been too late for this geopolitical turnabout.

The dominoes have, in the end, fallen. But it was the spread of capitalism, not communism, that knocked them over. China's combination of communist political rule and a market economy - where getting rich was applauded - has spread to Vietnam. As the two countries have shifted economic strategy, they have grown closer.

As I saw when I visited Vietnam last summer, the communist leadership may still be nervous about its giant Chinese neighbour - especially as China flexes its newfound economic muscle - but the two countries are deeply joined by burgeoning commerce. Border crossings are now clogged with trade and tourism.

Vietnamese officials and entrepreneurs were reluctant to openly criticise the Chinese. Vietnamese leaders, like others in the region, feel they must live with a rising China. They still want closer ties with the US, but they cannot afford to be drawn into a geopolitical alliance against China.

The Vietnam of 2005 remains a competitor for power with China, but the arena for competition is now business rather than the battlefield. Vietnam has a booming economy, drawing in foreign investors at an increasing pace. Consumers grab up everything from electronic gadgets to Paris fashions.

Three decades after "going communist," Vietnam is now fully a part of Asia, a member of the Association of Southeast Asian Nations (Asean) along with countries like Thailand and the Philippines that once were considered potential victims of a communist tide in the region. Instead of revolutionizing its neighbors, Vietnam is struggling with its own transition from communist rule to a more open society, facing the need to lift a legacy of repressive rule to keep its economy booming. And three decades after forcing the US out, Vietnam and its former enemy also are bound by commerce rather than geopolitics.

Vietnamese officials and entrepreneurs tour Silicon Valley, advertising their desire to be the next frontier of high-tech outsourcing. American airliners link San Francisco to Ho Chi Minh City, as Saigon was renamed at the end of the war. This Vietnam bears little resemblance to the one imagined on that neatly drawn map my father pointed to on that bright morning 40 years ago. The logic of the Vietnam War and the sense of purpose that seemed so certain to my father and me - and so many other Americans - has faded into history.

(Sneider is a foreign affairs columnist for the San Jose Mercury News. He can be reached at dsneider@mercurynews.com.)

--Indo-Asian News Service