The latest U.S.-Iran crisis has brought a hoary issue back to the center of the foreign policy debate: the question of credibility. Pundits have argued about whether the killing of Iran’s savviest military commander, Qassem Soleimani, will strengthen America’s reputation for forcefully defending its interests, and whether U.S. President Donald Trump’s prior restraint in the face of military provocations had invited Iran to act more boldly.

If the arguments seem familiar, that’s because similar debates erupted during the Syria “red line” incident of 2013, when U.S. President Barack Obama retreated from his threat to strike Syrian President Bashar Assad’s forces if they used chemical weapons against civilians. In fact, concerns about credibility go back to the beginning of America’s career as a superpower. And while the credibility debate has often pitted policymakers against academics, recent scholarship has helped bridge that divide by clarifying when and how credibility really matters.

The foreign policy community has long been obsessed with credibility, for good reason. Credibility — the perception that a country will defend its interests and uphold its commitments when challenged — is the coin of the realm for a global superpower. If U.S. credibility is strong, then it will not have to use force often, because other countries will recognize that attacking its interests and its global network of allies will invite sharp retaliation. A credible superpower has little trouble attracting and retaining allies. Yet if America’s credibility is weak, aggressors will be more likely to test U.S. power, and allies will be unnerved.

It is no exaggeration to say that American global influence and the stability of the international system rest on the credibility of U.S. threats and promises. For better or worse, the U.S. has even fought major wars in out-of-the-way places such as Korea and Vietnam in large part to convince allies and adversaries that it is willing to fight in more critical places, such as Europe.

Foreign-policy academics have taken a more conflicted view of credibility. A first wave of scholarship emerged in the 1960s, with the future Nobel Prize winner in economics Thomas Schelling at its crest. Schelling saw U.S. commitments as a seamless global web. In a contest against the Soviet Union, the way in which America responded to challenges anywhere would affect how other players gauged its willingness to respond to aggression everywhere. “We lost 30,000 dead in Korea to save face for the United States and the United Nations, not to save South Korea for the South Koreans, and it was undoubtedly worth it,” Schelling wrote.

Yet the fixation with credibility also brought strategic disasters such as Vietnam. So there subsequently emerged a second wave of scholarship, which held that past behavior was irrelevant to whether adversaries chose to challenge. Other variables, such as the balance of power and the importance of the interests at stake, were what mattered. As one scholar put it, “Credibility is an illusion — and an exceptionally dangerous one at that.”

This position was always a bit of a head-scratcher, because it required accepting that what a country does today has no impact on what others expect it will do tomorrow. Fortunately, a third wave of scholarship has now knocked down the more extreme academic critiques, without obscuring important nuances in how credibility actually works.

Cold War historians have suggested that U.S. intervention in Korea, which Soviet leader Josef Stalin had not expected, did strongly influence his views of whether Washington would also resist blatant communist military provocations elsewhere. We now know that John F. Kennedy’s perceived weakness in handling the botched Bay of Pigs invasion in 1961 encouraged Soviet leader Nikita Khrushchev to think he could bully the young U.S. president by trying to evict Western forces from Berlin and secretly shipping nuclear missiles to Cuba.

Looking beyond the Cold War context, America’s relatively timid responses to al-Qaida attacks during the 1990s encouraged Osama bin Laden to think that a major strike on the U.S. homeland might drive Washington out of the Middle East altogether. Likewise, a number of political science studies, many of them using statistical methods to divine broad trends, demonstrate that states that honor their commitments and meet challenges head-on are more likely to win allies and deter future challenges.

Why does any of this matter? For one thing, it shows that it is foolish to brush off concerns about credibility, as Obama did in remarking that “dropping bombs on someone to prove that you’re willing to drop bombs on someone is just about the worst reason to use force.” Especially in cases where the U.S. is interacting repeatedly with a single challenger, or where its reaction to one challenge might reasonably be expected to yield clues about its reaction to a similar type of provocation in the future, you can bet that Washington’s choices will shape global views of its credibility and resolve.

It seems almost certain that Trump’s reticence in using force against Iran through nearly all of 2019 influenced Tehran’s willingness to gradually increase the military pressure — and so it’s plausible that killing Soleimani may throw a wrench in the calculations of Iran or other Middle Eastern actors who are tempted to use force against American interests.

But don’t get carried away. Second-wave scholarship reminds us that context matters: The fact that the U.S. doesn’t use force to punish humanitarian outrages or protect a non-treaty ally in one region doesn’t necessarily have much impact on expectations of whether it will defend treaty allies from invasion in another region.

The situations just aren’t the same, because U.S. security interests and concerns about prestige are so much more intense in the latter case than the former. To use a historical analogy, the Soviets believed that the U.S. withdrawal from Vietnam signaled a declining American willingness to contest communist gains in the Third World. They didn’t think that Washington would remain passive in the face of a Warsaw Pact assault on the North Atlantic Treaty Organization.

Similarly, it is a stretch to suggest that U.S. inaction in Syria in 2013 tempted Putin to annex Crimea the following year. After all, the U.S. never had a treaty commitment to defend Ukraine, and local factors — namely, Putin’s nightmare of losing Ukraine to the West — surely overrode all other considerations.

Finally, credibility isn’t everything. If killing Soleimani convinces Iran that the U.S. means business, that’s good. But if it gets the U.S. thrown out of Iraq (thereby weakening its position against Tehran) or forces Washington to commit far more resources to the Gulf (thereby weakening its position against China), what has America really gained?

The debates surrounding credibility are complex. The good news is that this is a case in which recent academic work can help U.S. officials make sense of the complexity and avoid the mistakes that come with either undervaluing or overvaluing credibility. For policymakers, the scariest words in the English language need not be, “I’m from academia, and I’m here to help.”

Hal Brands is a Bloomberg Opinion columnist, the Henry Kissinger Distinguished Professor at Johns Hopkins University’s School of Advanced International Studies, and a scholar at the American Enterprise Institute.

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