Donald Trump’s presidency is forcing the foreign policy establishment to re-examine issues that had long been considered settled. A case in point is the US troop presence in South Korea.

Recent reports indicate that Trump has sought the withdrawal of some or all of the 28,000 US troops in South Korea, and has considered using the US presence as a bargaining chip in nuclear negotiations with North Korea.

Trump is not, however, the first president to scrutinize the American presence in South Korea. Jimmy Carter, for example, tried to withdraw American ground forces altogether. And the number of US troops has decreased over time, down from as many as 70,000 in the late 1950s.

So instead of being greeted by outrage, Trump’s inclination should raise a question that deserves a fuller answer: Why does America have troops in South Korea 65 years after the Korean War ended, and what does it get from the bargain?

The answer is that the benefits are indeed substantial, but also difficult to quantify, which is why the whole arrangement seems so unsatisfactory to Trump.

The strategic advantages of the US troop presence revolve around the twin imperatives of deterrence and reassurance. Deterrence involves keeping bad guys from doing bad things; reassurance means persuading good guys that the US will be there when needed.

Regarding deterrence, US troops help keep an aggressive North Korean regime in check. The Korean War started because Kim Il-sung, grandfather of the current leader Kim Jong-un, calculated that Washington would not intervene to stop him from conquering South Korea -- or that if it did, it could not arrive in time to make a difference. His miscalculation touched off a ghastly conflict that killed millions, including more than 30,000 US troops.

Thus the purpose of US troops in South Korea has been to show that America would be in the next Korean War from the outset -- that even if the North could somehow defeat South Korean forces, it would face the full might of the US.

Given that reunification of the peninsula is a stated goal of North Korean policy, given how brutal and violent North Korean policy has sometimes been even with US troops sitting across the demilitarized zone, and given that Pyongyang has been working relentlessly to develop increasingly advanced weapons, it is not hard to imagine things taking a turn for the worse absent the restraining effect of American presence.

Yet if US troops are there to restrain the enemy, they are also there to restrain America’s ally. In the aftermath of the Korean War, Washington officials worried that the South Korean dictator, Syngman Rhee, might mount his own campaign to forcibly reunify the peninsula. They therefore viewed the bilateral alliance and American troop presence as a way of keeping him from doing something dangerous.

Today, South Korea is a peaceful democracy, but the US troop presence still has a moderating effect on Seoul. By reassuring the South Koreans that Washington is fully committed to their defense, the American presence stifles the urge for Seoul to take other, more destabilizing steps to ensure its safety -- such as developing nuclear weapons of its own. When the American military appeared to be pulling back from Asia in the 1970s, in fact, South Korea took steps toward building a nuclear arsenal.

Since then, there has been a tacit bargain: The US maintains a tangible, visible commitment to South Korea, and South Korea foregoes the nuclear weapons it could easily develop.

The trouble, however, is that the benefits of this arrangement -- peace, stability, a climate conducive to commerce and prosperity -- are inherently nebulous. They are also inherently counterfactual, because they rely on judgments about the bad things that might happen if the US stopped doing what it does.

It is not surprising, then, that Trump doubts the value of the US presence -- because of his narrowly transactional mindset, and because he seems to lack the tragic imagination that would allow him to understand how a dangerous situation could get much, much worse absent American engagement.

What Trump sees are US troops are sitting in the middle of a potential war zone, helping defend a country that he deems an economic competitor as much as a strategic partner.

What he might be surprised to learn is that the US alliance and troop presence also provide narrower, more tangible benefits.

The US deployment makes both American and South Korean forces far more lethal in the event of war, by allowing them to train together constantly. It also provides leverage Washington can use to further other foreign policy objectives. South Korea sent forces overseas in support of America’s global war on terrorism in part to assure the US that Seoul was as committed to the alliance as Washington was -- the sort of transactional bargain Trump can surely appreciate.

And although Trump has often derided the US-South Korea free trade agreement, Washington got a better deal than the European Union did in similar negotiations, because Seoul was eager to please a country that contributes so much to its defense.

Finally, the cost of the US presence is considerably less than one might suspect. South Korea provides hundreds of millions of dollars each year ($765 million as of 2012) to defray the costs of US presence, arguably making it cheaper to station American forces there than stateside.

Clearly, then, Trump would be making a monumental mistake to pull US troops from South Korea. But he may be onto something in wondering whether there is anything sacrosanct about the current number and configuration.

Deterrence and reassurance are more an art than a science, because while the combat role American forces would play in a war is quite important, even more critical is the guarantee they represent: that more US troops would be coming if necessary.

It would surely take the presence of more than one US soldier to affirm the commitment to South Korea, and American forces should be capable of defending themselves and conducting effective operations. But whether the right number is 28,000 or 18,000 or 38,000 can be profitably reconsidered from time to time.

Whatever the precise number, though, the lesson of this episode is that those who believe that some substantial US presence on the Korean Peninsula is necessary will have to do better at explaining the benefits of that deployment. Because in the age of Trump, all the verities of US policy are up for debate.


Hal Brands
Hal Brands is the Henry A. Kissinger Distinguished Professor at the Henry A. Kissinger Center for Global Affairs at Johns Hopkins University’s School of Advanced International Studies and a senior fellow at the Center for Strategic and Budgetary Assessments. -- Ed.