Death Without Deterrence, or Why Tripwire Forces Are Not Enough
Many people think that U.S. troops deployed abroad have only one function: to die in the event of war. Before conflict breaks out, troops are often deployed abroad in smaller numbers, too few or ill-equipped to stop or slow down an adversary’s invasion. Instead, the thinking goes, because their deaths would enrage the American public and thus trigger broader U.S. intervention, the adversary won’t attack. In short, these militarily ornamental “tripwire” forces are thought to promise peace through body bags.
This is the wrong way to think about the role of U.S. forces based abroad. Militarily inconsequential forces are surprisingly ineffective at deterring aggression, as we argue in a recent article in Texas National Security Review. Potential attackers will still strike if they believe they can achieve their territorial goals swiftly, winning a fait accompli before larger reinforcements can arrive.
Even if tripwire forces are killed in combat as an aggressor achieves its goals, the public might not get outraged and may instead look to exit rather than risk more losses, as occurred in Lebanon in the early 1980s, Somalia in the early 1990s and Afghanistan in 2021 — in the last case even though American casualties have slowed to a trickle and withdrawal will likely usher in a Taliban victory. Or, even if the public supports mobilizing to overturn such conquest, the aggressors might presume a democratic aversion to bloodshed: that the population of the democratic adversary would not have the stomach for a longer, bloodier war if presented with a new territorial status quo. Japan in 1941, Argentina in 1982, and Iraq in 1990 all made the same error of miscalculating that American and British populations would cry for peace rather than scream for war.
Some have called for the deployment of American tripwire forces to Taiwan or the South China Sea to deter aggression by China. But attempting deterrence via body bags begs for miscalculation by China’s military planners, who may well assume that America would flinch in the face of casualties rather than escalating to push back against Chinese encroachment in either area. As one leading admiral in China’s navy declared in 2018, the United States would shy away from broader conflict with China, declaring “What the United States fear most is taking casualties … We’ll see how frightened America is.”
Supporters of tripwire deployments point to the successful deterrence of a Soviet attack on West Berlin as the critical episode demonstrating their power. As Thomas Schelling colorfully argued in the 1960s, “What can 7,000 American troops [in West Berlin] do, or 12,000 Allied troops? Bluntly, they can die.” The handful of Western troops deployed there were thought to have served as a deterrent because Moscow presumably recognized that their deaths would trigger a broader NATO response, not because they would actually stop a Soviet assault. However, we now know that this is a misleading account. Declassified archival sources indicate that NATO tripwire forces did not deter a Soviet attack because Moscow never intended to mount one. There was no Soviet desire to invade West Berlin, even in the crisis years of 1948–1949 and 1958–1961.
Though tripwire forces offer little deterrence power, larger or better-equipped troop contingents deployed to possible targets of aggression in peacetime can deter, if they are sufficient to shift the local balance of power. They do so by serving an operational purpose, not just as symbols of credibility. If deployed troops can stop, or even slow down, an aggressor’s attack — by fighting back, not by dying and triggering outrage — thereby blocking the achievement of a fait accompli, then an aggressor will lose confidence that it can win quickly. The fear of an attack becoming bogged down, and of risking opposition from an adversary undiscouraged by early casualties, should deter an aggressor from launching an attack. The end effect is the maintenance of peace.
The outbreak of the Korean War demonstrates our point. In 1949, Moscow and Beijing considered greenlighting a North Korean invasion of South Korea, but Joseph Stalin vetoed the idea because there was a sizable U.S. troop contingent deployed to South Korea. He saw that such forces would make a rapid North Korean conquest of South Korea unlikely.
However, the United States began to draw down its forces from 1949 into 1950, until only a smaller tripwire force remained. By spring 1950, Stalin became more confident that, with only a tripwire American force deployed to South Korea, North Korean forces could achieve a rapid fait accompli, conquering the entire peninsula before a broader American intervention could occur. He gave Kim Il Sung the green light, and North Korea invaded that June. The prospect of American combat deaths did not deter Stalin. Indeed, when he received early reports of American casualties following the invasion, rather than recoiling in discouragement, Stalin pressed North Korea to conquer South Korea quickly before broader American reinforcements could arrive.
These two basic points — that tripwire forces promise little deterrent effect but that larger forces can deter — speak to contemporary policy debates. The Trump presidency reopened debate about the financial costs of deploying U.S. troops abroad. Though Joe Biden is now in the White House, public fatigue with foreign commitments and fiscal pressures to focus on domestic spending might create a consensus for pursuing deterrence on the cheap. Deploying smaller troop contingents abroad would echo elements of the Eisenhower “New Look” foreign policy, under which sending smaller conventional forces to Europe was a means to keep defense spending down. Our analysis suggests that such budget-conscious policies would be penny-wise and pound-foolish.
Ongoing policy discussions sometimes embrace a certain tripwire pessimism, that all currently deployed troops can do is serve a tripwire function. A 2021 RAND report described U.S. forces deployed to South Korea as “modest,” allowing that the most likely scenario tempting a North Korean attack is if North Korea came to believe that it could conquer South Korea in a “seven day war,” before U.S. reinforcements could arrive. Separate analysis from the Modern War Institute at West Point agrees that North Korea would be most likely to attack if it came to believe it could conquer South Korea quickly.
America faces similar threats elsewhere. Russia poses a threat to the United States’ Baltic NATO allies, and some are concerned that the current “enhanced forward presence” defense plan can only serve as a tripwire and is insufficient to stop or slow down a Russian advance. And there are the threats to American interests that China poses, including territorial disputes with Japan, petrochemical ambitions in the South China Sea, and grander visions of conquest of Taiwan. Will U.S. force postures in these locations be sufficient to deny China’s forces the gains they seek, or will they rely on the logic (or hope) of a wider American response provoked if hostilities erupt?
American policymakers should ignore calls for cheap solutions to serious security threats. They need to assume that potential autocratic aggressors might come to doubt America’s willingness to intervene to overturn a territorial fait accompli, even if American troops from a tripwire force are killed in the attack. In peacetime, policymakers should deploy abroad troop contingents that, by the virtue of their front-line deployment, are guaranteed to fight and are large enough to shift the local balance of power and complicate attacker war plans by themselves. There are policy options that can shift the local balance of power in the Baltics and elsewhere. Let’s achieve peace by planning for victory rather than for human sacrifice.
Paul Poast, Ph.D., is an associate professor of political science at the University of Chicago and a non-resident fellow of the Chicago Council on Global Affairs. He is the author or co-author of three books, Economics of War, Organizing Democracy (with Johannes Urpelainen), and Arguing About Alliances.
Dan Reiter, Ph.D., is the Samuel Candler Dobbs Professor of Political Science at Emory University. He is the award-winning author, co-author, and editor of several books, including How Wars End, The Sword’s Other Edge: Trade-Offs in the Pursuit of Military Effectiveness, and Democracies at War (with Allan C. Stam).
Image: Gunnery Sgt. Rome M. Lazarus