Marines lace up their boots during mountain warfare drills with South Korean marines in Pohang, about 230 miles southeast of Seoul, in March 2010. (AFP/Getty Images)

For decades, tens of thousands of U.S. military personnel have been stationed in South Korea. This may change after the June 12 summit.

U.S.-North Korean negotiations over Pyongyang’s nuclear arsenal raise the possibility of at least a limited withdrawal, despite White House denials. Secretary of Defense Jim Mattis has hinted at post-agreement cutbacks. Other reports suggest President Trump has mentioned pulling U.S. troops out.

But U.S. troops also serve as a “tripwire force” in South Korea, so reducing their numbers could weaken U.S. security guarantees to South Korea. Without a U.S. military presence, some analysts suggest, the U.S. public might be less likely to support a U.S. intervention in the event of a crisis.

We recently investigated this claim in an online survey experiment. Our findings show that, at least in terms of their effect on public opinion, the importance of tripwires may be exaggerated.

Here’s why tripwire forces might matter for public opinion

The modern idea of a tripwire force dates to the Cold War. To deter Moscow, the United States and allies stationed troops in West Berlin — having those troops there committed NATO to escalate in the event of a Soviet attack. They were not expected to win. As strategist Thomas Schelling described their purpose, “Bluntly, they can die … heroically, dramatically, and in a manner that guarantees that the action cannot stop there.”

Analysts have endorsed Schelling’s claim that tripwires matter by affecting public opinion. Political scientists Joshua Rovner and Caitlin Talmadge described U.S. Cold War tripwire forces as “guaranteeing public support for defense of European allies.”

A recent Rand report suggested that if U.S. soldiers are present, “an attack could engage the U.S. public, which could add additional pressure on policymakers to respond with a larger U.S. force.” And in a classic article from 1997, political scientist James Fearon argued that leaders could tie their hands to intervene by using a tripwire force to make it costlier for them to back down.

Here’s how we did our research

The theory seems sound, but we don’t really know if it works — or how big of a response a tripped wire will generate. To find out how much the presence of a tripwire force matters for public opinion, we turned to conjoint analysis. This is a type of survey design that asks respondents to assess paired scenarios in which relevant characteristics randomly take on different values.

We presented 1,200 participants on Amazon’s Mechanical Turk platform with five pairs of scenarios in which an aggressor country attacked a target country. We randomly varied the values of 15 characteristics in the scenario. This let us simultaneously estimate the effect of a large handful of variables — including the presence and size of a tripwire force — on support for intervention.

Our experiment dealt with scenarios in which tripwire forces may have been present and could easily be drawn into a conflict but had not been directly attacked. Understanding the effect of a direct attack on U.S. forces stationed abroad is an important next step, but it requires first exploring whether the presence of troops works as a tripwire.

A larger force revealed only a slight increase in public support

Our results show that the presence of larger tripwire forces modestly increases support for intervention. Respondents were only about three percentage points likelier to support an intervention when they read that tens of thousands of U.S. troops were stationed inside the victim country, compared with when there were no troops present. A smaller force of a few thousand also increased support by about three percentage points, compared with the no-troops condition.

To put that into perspective, the presence of an alliance treaty between the United States and the victim also increased support by about five percentage points, as did U.N. Security Council approval. A large tripwire force, in other words, is slightly less important (or, given statistical uncertainty, about as important) as these diplomatic measures.

All of those factors, however, were far less meaningful than the likelihood of success and the expected levels of casualties the United States would sustain. The probability that an individual would support intervention was roughly 15 percentage points lower when respondents read that the operation would probably not succeed, compared with when they read that an intervention would almost certainly succeed. Even giving an intervention roughly even odds of succeeding decreased support by about six percentage points relative to almost certain success.

The costs of war mattered greatly, too. Support for an intervention declined by about 17 percentage points when more than 10,000 U.S. military casualties were expected, compared with interventions in which several hundred casualties were expected. Even moderate casualties of several thousand reduced support by about eight percentage points. This finding is significant in light of work by Tanisha Fazal, reported here in the Monkey Cage. Fazal’s findings suggest that U.S. casualties would be higher in a renewed Korean War than in recent U.S. conflicts.

Tripwire forces may not trip the wire of public opinion

There is still much to learn about tripwires and the political effects of other forward-deployed military assets. Perhaps the deaths of U.S. troops would produce a more supportive public response or ease congressional authorization, for instance. In that case, tripwire forces might have a stronger effect on support for escalation than our findings suggest. 

But our preliminary findings imply that an attack on South Korea with U.S. forces present may not trip the wire of public opinion as dramatically as often claimed. Rather, our experiment found that the most important drivers of U.S. support for a U.S. intervention are expectations about battlefield outcomes and costs.

Given the pessimistic and high-casualty expectations for any new war on the Korean Peninsula, the U.S. public may not be eager for war.

Steven Ward is an assistant professor in the Government Department at Cornell University and Carnegie Junior Faculty Fellow at Stanford University’s Center for International Security and Cooperation. 

Paul Musgrave (@profmusgrave) is an assistant professor of political science at the University of Massachusetts-Amherst.