A Reuters article suggested last week that the United States is considering withdrawing as many as 4,000 troops from South Korea if Seoul does not pay more money to maintain the U.S. deployment there. The Pentagon denied the report, which appeared to echo other reports that President Trump demanded greater contributions from South Korea and Japan to support the U.S. military presence in those countries.

Is the 66-year-old alliance with South Korea in “deep trouble,” as one Washington Post editorial suggests? A Korea Times headline said it bluntly: “Put alliance before money.”

Debates over burden sharing — how the United States and its allies split the costs of providing security — date to the beginning of the Cold War, but Trump’s moves could have major consequences for U.S. alliance relationships. Here’s what you need to know.

What do allies contribute?

It’s not news that Trump wants allies to pay more to compensate the United States for its protection. Trump’s antagonism toward NATO members has received the most attention, but he has also pressured U.S. allies in the Asia-Pacific region. The big news now is that Trump reportedly asked South Korea and Japan to boost their annual contributions by 400 percent or more, in addition to the reported threat to withdraw U.S. military units. Previous administrations have tried to persuade allies to spend more money on defense, but Trump’s move is a sharp departure from the past 70 years of U.S. foreign policy.

U.S. allies routinely help offset the substantial costs of developing and maintaining the U.S. security overseas presence. The figure below shows Department of Defense estimates of the cost to maintain the U.S. presence in select countries. The estimate for fiscal 2020 is $5.7 billion in Japan and $4.5 billion in South Korea.

The U.S. government regularly renegotiates these burden-sharing arrangements with host countries. The 2019 cost-sharing agreement requires South Korea to pay approximately $893 million in 2019 to help defray the cost of sustaining the U.S. military’s presence there. This amounts to about 20 percent of the DOD’s total estimated costs in South Korea.

Japan’s most recent agreement, from 2014, is less detailed but still provides for substantial contributions. Archived Pentagon reports indicate that Japan covered approximately 75 percent of costs in 2002. However, these estimates often do not count expenditures toward new construction projects or training exercises and typically exclude expenditures on weapons systems manufactured in the United States — so total “contributions” are often higher.

Allies are unhappy

The fact that U.S. allies already make substantial contributions means that they are unlikely to react well to the Trump administration’s demands for such sharp increases in burden sharing, unless there is a serious justification. The reporting on the South Korea price tag indicates that Trump’s demands came “out of thin air,” leaving DOD officials struggling to justify the $4.7 billion figure.

Analysts note the Trump demands may lead allied leaders to question the reliability of their U.S. partners and start exploring alternative means of providing security. In fact, South Korea and China recently signed an agreement to strengthen their military and security ties. And citizens of allied countries may be unhappy with transferring so much money to the U.S. military.

Surveys we conducted in 2018 indicate that about 35 percent of Japanese citizens and 53 percent of their South Koreans counterparts hold positive views of the U.S. military personnel deployed in those countries. Similarly, 30 percent of Japanese and 46 percent in South Korea hold positive views of the U.S. government.

However, we also find that 30 and 45 percent of the Japanese and Korean public, respectively, thinks the U.S. presence has a positive impact on the national economy. And these figures are considerably lower when we ask about the effect on local economies.

The U.S. presence has generated political controversy in the past: 81 percent of Japanese respondents and 52 percent of South Korean respondents view the U.S. military presence in their country as a “somewhat divisive” or “very divisive” issue. A sharp uptick in U.S. demands for more money may reignite domestic opposition to the U.S. military presence.

Free riding can have benefits

Do allies take a “free ride” on the U.S. provision of security? Our work on the topic of burden sharing and U.S. troop deployments find that a larger U.S. military presence correlates with less defense spending by the host country. However, the U.S. receives greater influence in return: Countries relying on the U.S. military are less able to pursue their own adventurous foreign policies.

Political scientists refer to this dynamic as the “security/autonomy” or “security/sovereignty” trade-off. The United States provides security to other countries. And in exchange, those countries are limited, either by agreement or by reduced capabilities, from pursuing the full range of foreign policies that they might otherwise pursue. Put simply, a security relationship with the United States comes with conditions, and those conditions give the United States greater say in what happens around the globe, and greater ability to project force in response to unexpected crises.

The United States also gains substantial benefits to its own foreign policy from its “forward” position in other countries. For example, it also allows the U.S. military to respond more rapidly to emerging crises, acts of aggression or natural disasters.

Trump’s threat to withdraw U.S. forces from allied states may prompt longtime allies to rethink their relationships with the United States and start to develop their own defense capabilities. This might lower the financial costs to the United States — but would also lower the U.S. ability to influence other states. The result, perhaps, will be a more militarized world and a higher chance of international conflict.

Parts of this article are based on work supported by, or in part by, the Minerva Research Initiative, the U.S. Army Research Laboratory, and the U.S. Army Research Office under grant number W911NF-18-1-0087. The opinions and interpretations are those of the authors and not the U.S. Army or Department of Defense.

Michael Flynn is an associate professor of political science at Kansas State University. His research focuses on the political economy of foreign policy and security issues, as well as military deployment. You can find him on Twitter at @michael_e_flynn

Michael Allen is an associate professor of political science at Boise State University. His research focuses on asymmetric relations, hegemony, and the positive and negative externalities of foreign policy. You can find him on Twitter at @michaelallen

Carla Martinez Machain is an associate professor of political science at Kansas State University. Her research explores military effectiveness and public perceptions of the military. You can find her on Twitter at @carlammm