Dilemmas of Deterrence

We live in a world where geopolitical stability relies largely on deterrence. But how can we prove that deterrence works?

Consider the ongoing war in Europe. Beginning in December 2021, US President Joe Biden warned Russian President Vladimir Putin that Russia would face severe new sanctions if he invaded Ukraine, to no avail. Then, when the United States and its European allies thwarted Russia's plans by providing arms to Ukraine, Putin brandished the nuclear option. But Western aid continued unabated.

Did deterrence fail or succeed? Answering this question poses a challenge because it requires assessing what would have occurred absent the threat. It is hard to prove a negative. If I put a sign on my front door saying, "No Elephants," and there are no elephants in my house, did I deter them? It depends on the likelihood of literate elephants entering in the first place.

The Ukraine war demonstrates how risk reduction is not always an either/or choice, but often a matter of degree. Perhaps Putin, counting on a flimsy Western alliance, believed the sanctions would fail. But he has so far refrained from striking supply lines in NATO countries. And while the West, for its part, has continued to arm Ukraine despite Putin's nuclear saber-rattling, it has been reluctant to provide longer-range missile systems or modern warplanes.

Credibility is essential for deterrence to work: threatening a maximum response to defend a minor interest strains credulity. This is particularly true when a nuclear power promises to extend its umbrella to defend a distant country....