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. Those reports have alarmed foreign policy experts in both political parties, who cite it as one more example of Trump’s geopolitical recklessness.

Trump is not, however, the first president to scrutinise 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. 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, with unmistakable clarity, 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.

It is impossible to know, of course, whether Pyongyang would be so bold as to attack South Korea absent the US presence. But 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 demilitarised 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. 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 defence, the American presence stifles the urge for Seoul to take other, more destabilising 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 US troop presence is the key to deterrence and reassurance on the Korean Peninsula, bringing tenuous stability to an important part of the world.

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 judgements about the bad things that might happen if the US stopped doing what it does.

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 defence.

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.

When a French general was asked, over a century ago, how many Englishmen would be necessary to defend France, he replied, “One single private soldier, and we would take good care that he was killed.” The idea was that even a single British casualty would cause an outraged Britain to rally to its ally’s defence.

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.

— Washington Post

Hal Brands is a 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. His latest book is “American Grand Strategy in the Age of Trump”.