written by Todd DePastino

Vintage map showing China's invasion of Vietnam during the Third Indochina War

On February 17, 1979, China launched over 200,000 soldiers across its southern border in a “punitive war” against Vietnam that would last three weeks and cost tens-of-thousands of lives on both sides.

The so-called “Third Indochina War” or “Sino-Vietnam War of 1979” was, for Americans, a perplexing coda to the Fall of Saigon. Why would Vietnam’s staunch ally to its north, a fellow Communist country, suddenly turn against its former comrade-in-arms?

The story has its roots in the long and complicated history of Chinese-Vietnamese relations. And, as you consider the causes of the war, it’s good to keep in mind Clausewitz’s famous dictum that war is merely “politics by other means.”

When Ho Chi Minh launched his war of independence against the French, Communist China, along with the Soviet Union, became Ho’s most important strategic ally. It’s hard to imagine Ho’s forces winning in 1954 without the arms supplied by China.

That support increased in the American War after 1964, when China sent troops to Vietnam to man anti-aircraft batteries and train the North Vietnamese Army. They also gave important economic aid to keep the nation afloat during the all-out war to reunify North and South Vietnam.

Relations between the two countries began to sour on April 30, 1975, the day North Vietnam finally triumphed over the South and brought the two halves together to form the Socialist Republic of Vietnam.

China, it turns out, wasn’t thrilled by the Vietnamese victory. And Vietnam, for its part, was wary of China’s involvement in its affairs, even as it depended on Chinese support.

If there’s one thing we’ve learned from the bloody 20th century, it’s that geography almost always trumps ideology. Ideology wrecks havoc on the world, but after the bloodletting, nations tend to revert to the mean. That is, they return to strategic positions that have more to do with borders, land masses, sea lanes, and topography than grand ideas about how the world should be ordered.

Vietnam and China shared a Communist ideology, but, in the end, the mountainous border between the two countries proved more important.

That’s because back in 111 BCE, the Han Dynasty had invaded Vietnam and ruled it for 1,000 years. In fact, the very name Viet Nam was given by the Chinese to denote “the Land Beyond the South.” The Vietnamese fought a 1,000-year insurgency against their Chinese overlords, winning independence in 938 CE and staving off several subsequent invasions over the next 1,000 years.

In the 1950s and 1960s, when China so generously assisted Vietnam by placing tens of thousands of advisors and support troops in the country, it was really seeking to exercise control over a territory it had long considered properly belonging to China.

The Chinese saw the Vietnamese as a junior partner. When Vietnam lacked sufficient gratitude for China’s support, China felt the affront.

But affronts alone don’t usually mean war.

What triggered the armed clash on Vietnam’s northern border in 1979 was, in fact, another matter of geography: Vietnam’s invasion of Cambodia on Christmas Day 1978

On the surface, Vietnam and Cambodia should have been allies. Communists had swept into power there at the same time as the Fall of Saigon in 1975. With the triumph of the Communist insurgency in Laos the same year, it meant that all of Indochina, plus China itself, were Communist. The dominos had fallen.

Color map of Russian and East Asia

But, in another example of geography triumphing over ideology, Pol Pot and the Communist Party of Kampuchea, the formal name of the Khmer Rouge, feared that Vietnam was attempting a takeover of Cambodia. China was to Vietnam what Vietnam was to Cambodia.

In fact, Vietnam for centuries had considered the southeastern part of Cambodia as properly Vietnamese. In the 1750s, the Dai Viet (“Great Viet”) empire conquered the Mekong Delta and claimed control over the Khmer population living there.

In a what’s-good-for-the-goose-is-good-for-the-gander turnabout, the Khmer Rouge began attacking Vietnam, both to reclaim lost territory and stave off any attempt by the Vietnamese to assert itself as ruler over all of Indochina.

China backed Cambodia and the Khmer Rouge in this growing clash between neighboring Communist states. On December 25, 1978, Vietnam declared it had enough of the Khmer Rouge’s assaults and launched a full-scale invasion of Cambodia, swiftly toppling Pol Pot’s government there.

That’s when China’s paramount leader, Deng Xiaoping, decided to teach Vietnam a lesson. On January 29, 1979, during a visit to the United States, Deng told US President Jimmy Carter, “the child is getting naughty, it is time he got spanked.”

Three weeks later, China’s People’s Liberation Army (PLA) poured into Vietnam, launching the Third Indochina War.

Americans, if they thought of it at all, were confused about this strange turn of events a half-a-world away. But China’s gambit was part of its larger warm-up to the West, especially the United States. By attacking the US’s erstwhile enemy, the Chinese hoped to score points with the United States and send a message to a menacing new adversary: the Soviet Union.

It was geography-over-ideology once again. China and Russia had been best of friends after Communist Mao Zedong came to power in Beijing in 1949. But the alliance didn’t last. By 1961, the two Communist powers were facing off, as their imperial predecessors had in previous centuries. No amount of Marxist-Leninist doctrine could erase the tension built into the 2,500-mile Sino-Russian border.

The “Sino-Soviet Split” placed North Vietnam in the difficult position of choosing which parent they preferred. By 1968, the North Vietnamese took the Soviet Union’s side, and China began withdrawing its support of Hanoi. Vietnam’s reunification in 1975 gave the Soviet Union a larger foothold in Indochina. The Soviet-Vietnamese alliance was a threat to Chinese hegemony over Southeast Asia.

Thus, the Chinese invasion of Vietnam in 1979 was not a sudden, isolated event but rather a culmination of grievances and part of a larger strategic struggle between Great Powers. Deng himself declared he was launching the war against “dogs of the Soviet Union.”

The fighting of the Third Indochina War was brief but bloody. China launched a two-stage offensive, targeting the border cities of Cao Bang, Lao Cai, and Lang Son. Vietnam, caught off guard by the invasion, mobilized local militias and used classic defensive guerrilla tactics–jungle warfare, tunnel systems, and fortified positions–until regular Vietnamese forces could be moved from Cambodia to support the defense.

After 28 days of human wave attacks and scorched earth tactics, China withdrew to the border area and claimed victory.

Vietnam, on the other hand, boasted about driving the invader out.

Beijing was satisfied it had proved its point, scored a propaganda victory over the Soviet Union, and demonstrated its ability to keep Vietnamese ambitions and pretensions in check. The limited time frame of the conflict prevented it from escalating into a larger Sino-Soviet war.

With no independent news outlets on the ground during the fighting, the war’s casualty estimates range wildly. The best guess is that between 50,000 – 60,000 people on both sides died in the three-and-a-half weeks of fighting, a jaw-dropping indication of military incompetence and the disregard of human costs.

Today, the Sino-Vietnamese War of 1979 is little remembered, even in China and Vietnam. But the core dynamic between the two countries remain. In a richly ironic role reversal in this Great Power play, the United States is now aligned with Vietnam, while China and Russia support each other. The next time China seeks to spank its naughty child, it will await response from the US 7th Fleet in the South China Sea.