Does momentum matter in global affairs? If one country falls to hostile forces, will others follow?

The domino theory — the idea that setbacks can accumulate quickly and catastrophically — is one of the most controversial ideas in U.S. foreign policy. It guided a half-century of statecraft during the Cold War, only to be rejected by many historians of that conflict. But the anniversary of the Korean War, which began 70 years ago on June 25, 1950, reminds us that the domino theory was rooted in key realities of international affairs. And it alerts us that domino dynamics could affect U.S.-China competition today.

The domino theory became infamous thanks to the Vietnam War, which the United States fought in part to prevent the rest of Southeast Asia from falling. U.S. President Dwight D. Eisenhower coined the phrase in 1954, in explaining why losing Indochina would be so disastrous: “You have a row of dominoes set up, you knock over the first one, and what will happen to the last one is … it will go over very quickly.” Beyond Vietnam, the idea that one defeat could trigger others was ubiquitous during the Cold War.
A principal reason U.S. President Harry Truman provided aid to Greece and Turkey in 1947 was fear that other parts of the Middle East and Europe would find it impossible to resist if these countries succumbed to Soviet pressure or communist subversion. The Korean War was fought on the same premise. If the U.S. failed to defend South Korea, the result would be demoralization and neutralism in Japan and Western Europe. In the 1980s, the U.S. waged a decade-long covert war in Nicaragua to prevent the Sandinista revolution from turning the rest of Central America red. The balance of power was fragile, American officials believed, and even a few cracks could cause it to shatter.

Those looking back at the era have mostly taken a dimmer view of the domino theory, seeing it as sloppy strategic thinking or simply Cold War hysteria. But there was something to the fear of falling dominoes, and Korea reveals why.

There were other mini-domino effects elsewhere. When Vietnam fell in 1975, Laos and Cambodia went with it. The Soviets pushed harder elsewhere along the global periphery, in places such as Angola. U.S. allies in Southeast Asia had doubts about Washington’s staying power. In Latin America, the Cuban revolution in 1959 incited communist insurgencies in countries throughout the region, in part because of the aid and inspiration Fidel Castro provided.

Two decades later, the Sandinista victory in Nicaragua did not directly cause revolutions in Guatemala and El Salvador, where communist movements were leading movements against highly repressive regimes. But it did increase the intensity of the violence in Guatemala and especially El Salvador, as the Nicaraguan government created a logistical hub for other Central American revolutions.

As these examples showed, a communist takeover in one country would allow that country to put pressure on its neighbors, through subversion or support for outright aggression. The domino theory simply reflected the psychological and geopolitical realities of competition in an interconnected world. To the extent that the feared mega-domino effect never happened, it was most likely because the U.S. and its allies invested so much in shoring up positions under strain, as Washington did in Korea in 1950.

Those interventions weren’t always wise. The U.S. war in Vietnam may have bought time for other parts of Southeast Asia, but the price, in human and strategic damage, was astronomical for America itself. The Korean War also turned very bad, after the Truman administration’s effort to liberate North Korea brought on Chinese intervention. The domino effect was real, but how to respond was still a matter that required good judgment.

Strategists in the new cold war should pay heed. China may not be bent on outright conquest of its neighbors; it doesn’t command the loyalty of communist parties around the world the way the Soviet Union often did. Yet there are still domino dynamics that could affect key areas of the competition.

Taiwan, for example. The island occupies a strange place in U.S. policy. Washington does not recognize it as a country but has an ambiguous commitment to defend it from Chinese aggression. Some prominent analysts have recently suggested that simply allowing Beijing to reabsorb its “renegade province” would remove a major irritant in the U.S.-China relationship. But leaving aside the moral implications of such a policy, the geopolitical implications would be terrible.

If China took control over Taiwan, it would undermine U.S. strategy throughout the region. The disputed Senkaku Islands would became far harder for Japan to defend. China would have a much easier path into the open Pacific. The diplomatic ripple effects would surely unsettle other exposed countries that depend on American power. Beijing wouldn’t be able to simply snap up the rest of the Asia-Pacific, even if it wanted to. But the loss of one crucial position would have the effect of endangering others.

More broadly, if countries in the Asia-Pacific or elsewhere believe China is getting the upper hand in its struggle with the U.S., they will be less inclined to run the risks of opposing it. Lose in a few places, and people in lots of other places might think you can’t win.

This doesn’t mean the U.S. should go all-out to compete with China everywhere, just as it wasn’t prudent to fight a massive land war in Vietnam. Some places don’t actually matter that much: Victory in Angola didn’t bring the Soviets any closer to victory in the Cold War, and winning influence in some corrupt country in Central Asia or sub-Saharan Africa might not help China much in the long run. But global competition does have a way of bringing out the interlinked nature of the strategic environment, and reminding us that what happens in one country may not stay there.

Hal Brands is a Bloomberg Opinion columnist, the Henry Kissinger Distinguished Professor at Johns Hopkins University’s School of Advanced International Studies and a scholar at the American Enterprise Institute. Most recently, he is the co-author of "The Lessons of Tragedy: Statecraft and World Order."

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