Joshua Keating asks today, "what does deterrence even mean, really?" It's a good question for at least two reasons: (a) deterrence against what? and (b) what's the evidence?
The first question is critical. The most famous case of deterrence is the MAD doctrine during the Cold War. It worked! But in the modern world deterrence more commonly means the ability to stop actions short of war, such as terrorism, limited missile attacks, and so forth. The most recent example of this is a clear failure: Hamas's October 7 attack on Israel obviously wasn't deterred even though Israel has a fearsome reputation for revenge.
So even if we accept that countries think twice about starting wars they might not be able to win—like, say, Lebanon invading Israel—there's still the smaller stuff to consider. This is where evidence becomes so important.
This is pretty easy to understand. For example, Donald Trump assassinated Qasem Soleimani, the head of Iran's Quds force, and his supporters like to say this kept things quiet for a while. But did it? The evidence is pretty equivocal:
Days after the attack, Iran retaliated by firing about a dozen ballistic missiles at two U.S. bases in Iraq. While no U.S. service member was killed, more than a hundred American troops suffered brain injuries as a result of the Iranian response.
....“There have been subsequent incidents including in March when Iran-backed Iraqi militia rockets killed two Americans and a Brit. The Iranians also used their small boats to swarm U.S. Naval vessels in April — without any apparent U.S. response,” [Barbara Slavin] said.
So maybe it worked and maybe it didn't. Can you ever really know? This gets us to the big problem of empirical deterrence research. If something happens, it's easy to say that it wasn't deterred. But if nothing happens, does it mean something was deterred? That's much harder to say.
This sets up a huge bias in favor of ever more deterrence. You can always point to attacks as evidence that we need stronger deterrence, but it's exponentially harder to point to nothing—or, harder still, a little bit of something but not very much—as evidence that we already have plenty of deterrence.
But after 9/11, the US made no mystery of its purpose. It was not to deter al-Qaeda, it was to destroy al-Qaeda. Ditto for Israel after the Hamas attack. In both cases, the countries involved decided that their foe could not be deterred.
And that seems to be the case generally for guerilla wars, insurgencies, and terrorist attacks. Even the threat of massive retaliation doesn't stop them. Nation states can be deterred from flatly starting a war with a superior power, but neither they nor anyone else show much potential for being deterred from anything else.