US officials are always talking about “deterring” Iran. What does that really mean?

Does the US still have the power to deter its adversaries?

When the US launched airstrikes earlier this month against the proxy militias linked to Iran’s Revolutionary Guard Corps (IRGC) in retaliation for the attack that killed three US soldiers at a base in Jordan, President Joe Biden noted that while the US would continue to respond to Iran-linked attacks “at times and places of our choosing,” it “does not seek conflict in the Middle East or anywhere else.” Secretary of Defense Lloyd Austin’s statement on the strikes used almost identical language.

Readers of these statements might reasonably wonder how one can bomb 85 targets and kill nearly 40 people without “seeking conflict.” B-1 bombers are not exactly an instrument of Gandhian nonviolent resistance. Officially, the motivation for the strikes, according to the letter the White House sent to notify Congress in accordance with the War Powers Resolution, was to “deter the IRGC and affiliated militia groups from conducting or supporting further attacks on United States personnel and facilities.”

Since the October 7 terrorist attacks in Israel and the war in Gaza that followed, “deterrence” — specifically deterring Iran and its various proxies from initiating a wider regional war — has been the guiding concept behind US policy. “Deterring a broader conflict” was cited by Pentagon officials as the motivation for deploying more US military assets, including aircraft carriers, to the Middle East last fall. In January, the US launched military strikes in Yemen with the goal of “deterring Houthi attacks in the Red Sea.”

This sort of language is not new. Establishing a “strong deterrent” against chemical weapons use was President Donald Trump’s stated motivation for launching airstrikes against Bashar al-Assad’s regime in Syria in 2018. President Barack Obama assured wary American allies in the Persian Gulf in 2016 that despite his attempts to reach a nuclear deal with Iran, he would not hesitate to use military force to “deter” Iranian aggression.

In recent cases, at least, the effectiveness of all this deterrent activity has been decidedly mixed. Iran has not yet directly attacked Israel or US military targets with its own forces. Lebanon-based Hezbollah, Iran’s most powerful proxy, has not yet launched a full-scale war with Israel, as many feared earlier in the conflict. But at the same time, US troops in the region have been targeted in dozens of attacks that have resulted in those three deaths and dozens of wounded; the exchange of fire between Hezbollah and Israel has been intensifying, even if it’s still short of all-out war; and the Iran-backed Houthis have continued their attacks on shipping, declaring that Western airstrikes “will not deter us.” Just two days after the US strikes in early February, a drone attack claimed by Iran-linked militias against a US base in Syria killed six Kurdish fighters, who are allied with US forces in the region.

At press briefings, US government spokespeople now regularly face questions about whether US deterrence in the Middle East has failed. But with conflicts on the rise globally and many longtime partners starting to question the value of US security guarantees, it’s a question that has ramifications beyond just this region. In today’s world, is the US still able to deter its adversaries?

Carrying a big stick

The concept of deterrence — dissuading an adversary from carrying out some action through the threat of punishment — has been a feature of international relations and military strategy since at least the ancient Greeks. The concept was fleshed out at the height of the Cold War by theorists like the Nobel Prize-winning economist Thomas Schelling, whose 1966 book Arms and Influence examined how countries can use military power — including, in the nuclear age, the power to kill millions almost instantaneously — to influence each other’s behavior.

Schelling portrayed superpower competition in the nuclear era as a “competition in risk taking, characterized not so much by tests of force as by tests of nerve.” In other words, the measure of a country’s power to deter or coerce its adversaries was not just the number of troops, guns, and bombs at its disposal but its ability to create the impression that it is willing to use them, a dynamic he refers to as the “manipulation of risk.”

The ongoing war in Ukraine has been a textbook lesson in the manipulation of risk. Throughout the conflict, Russia has had to tolerate billions of dollars worth of Western military aid flowing into Ukraine, significantly bolstering that country’s ability to fight. Critically, this aid is flowing from NATO countries like Poland, which are covered by a treaty that declares an attack on any member of the alliance to be an attack on all. Vladimir Putin evidently considers that guarantee credible and wants to avoid getting into a shooting war with the entire alliance by attacking the aid shipments.

But deterrence cuts both ways. NATO countries want to avoid a direct war with Russia, too, particularly in light of Putin’s frequent threats to use nuclear weapons. This has limited the types of assistance they have provided to Ukraine: no NATO troops on the ground, no NATO planes enforcing a no-fly zone over Ukraine.

Over time, though, the West has gradually increased the amount of aid it has provided to Ukraine, with no catastrophic nuclear response from Russia, undermining the credibility of Putin’s threats. So Ukraine is now receiving forms of aid — targeting assistance, tanks, fighter jets — that would have been unthinkable in the early days of the war, when according to many accounts keeping the fighting contained within Ukraine was a bigger priority for US security officials than Ukrainian victory. This type of escalation — gradual but without ever doing anything dramatic enough to put your adversary in a position where they feel compelled to respond — is referred to, by Schelling at others, as “Salami tactics,” meaning cutting off one thin slice at a time, eroding your opponent’s red lines without provoking them into a major response.

What has not been deterred, of course, is Russia’s willingness to prosecute ongoing, extraordinarily destructive war within Ukraine itself.

The limits of deterrence

Bilal Saab, a former Pentagon official now with the Middle East Institute, says it’s not accurate or fair to say that the ramped-up US military presence in the Middle East has failed to deter Iran. “If there weren’t that forward deployed immediate deterrence in the region, you probably would have seen a whole lot more activity from Iran to spread its influence without being checked,” he told Vox. Saab pointed out some far more provocative actions that Iran could have taken, including an attack that killed dozens rather than just a few US soldiers or shutting down the Strait of Hormuz, which would have an even more significant impact on global energy supplies than the disruptions the Houthis have caused in their attacks on Red Sea shipping.

Saab acknowledged that it’s difficult to “prove that deterrence is working because at the end of the day, you can’t prove something that didn’t happen.”

At the same time, the frequent attacks by Iranian proxy groups against US military targets in the Middle East — more than 160 strikes since October, according to the Foundation for the Defense of Democracies — show that there are some activities Tehran has definitely not been deterred from carrying out.

“It’s basically become the status quo: a proxy group lobs rockets at US bases, then US forces bomb and kill the proxies,” said Emma Ashford, a senior fellow at the Stimson Center.

Even if this state of affairs avoids a catastrophic regional war, it leaves US troops in the region — notably the roughly 2,500 in Iraq and 900 in Syria who are ostensibly there to defeat ISIS — in the crossfire. As the Economist recently noted, these troops constitute a “military presence big enough to present a menu of targets but too small actually to constrain Iran.”

Jonathan Lord, a former Defense Department official who now directs the Middle East program at the Center for a New American Security, said the attacks on these targets seem calculated to “inject uncertainty into either US or Israeli decision making” but “avoid putting the US to a decision point where it has to respond forcefully.” Both sides are trying to influence the other’s behavior without sparking a conflict they can’t control. Using proxies makes it easier to maintain this balance than it would be if Iran were directly attacking the US military with its own military.

Of course, even “controlled” tit-for-tat exchanges of potentially deadly fire can easily lead to unintended escalation. The strike in late January that killed three US troops may have been a case of the “dog that caught the car,” Lord said, in that it prompted the US to take much more serious action in retaliation.

Some Republican critics have called for the Biden administration to take even more aggressive action to deter Iran, including striking within the country itself. Given the track record of US military interventions in the region over the past 20 years, the administration has very good reason to avoid getting involved in a direct conflict with Iran. (For what it’s worth, Trump also stopped short of striking within Iran itself, despite coming very close to doing so.)

Lord said that the US reluctance to escalate is “coming from a good place and well-intentioned,” but that the strategy has essentially given Iran the message that a certain amount of violence targeting US troops will be tolerated, or at least that the response to it will be measured and limited. “[Iran] has a freer hand, knowing that we’re going to be moderating our actions,” he said.

The US retaliation to the Jordan strike that killed three troops does appear to have had some effect. Iran has reportedly instructed the militia behind the attack, Kataib Hezbollah, to stand down, and the number of overall strikes has since decreased. But it’s far from clear how long this will last. The US drone strike in 2020 that killed Qassem Soleimani, commander of Iran’s Quds Force and architect of its proxy network, was also supposed to restore “deterrence” against Iran. In fact, it may have made some of the proxy groups more aggressive and less predictable since Tehran’s direct control over them does not appear to be as strong as it once was. It’s another factor that makes deterrence less reliable — proxy forces may not operate under the same assumptions as their sponsors.

Ashford was skeptical of the notion that military strikes against these groups constitute “deterrence” at all, in the traditional definition, noting that the whole point of deterrence is to prevent your opponent from taking some action, not responding to them after they already have. Once you’re exchanging fire with your adversary, that adversary has, by definition, not been deterred.

“Either have deterrence or you don’t,” she said. “It’s either succeeding or it’s failed. Maybe you could have deterrence again in the future, but there’s no such thing as ‘restoring’ deterrence.”

The future of deterrence

The Middle East is not the only place where US deterrence has been called into question in recent days. At a rally in South Carolina last week, former President and current candidate Trump said he would encourage Russia to “do whatever the hell they want” to NATO countries that failed to meet the alliance’s defense spending targets. Trump is not alone in calling for some European countries to meet those targets, but NATO isn’t like a country club where you have to pay your dues to get your service. With his comments, Trump fundamentally called into question the very mutual defense guarantee that has, so far, successfully deterred Russia from directly attacking NATO states.

The ability to use the threat of military force to prevent an ally from attacking not only your own territory but the territory of your allies is often called “extended deterrence,” but it only works if those allies and adversaries are certain you will live up to your commitments. That credibility is tough to maintain in today’s hyperpartisan US, where — as shown by Congress’s current inability to pass aid packages for Ukraine, Taiwan, and Israel — there’s little consensus on basic national security priorities or continuity between administrations.

“Any country that’s in a treaty relationship, or in a treaty alliance with the United States can no longer treat Washington as a somewhat predictable being,” said Ankit Panda, a senior fellow in the nuclear policy program at the Carnegie Endowment for International Peace. Allies know that Biden will, as he promised, defend “every inch” of NATO territory — as President Obama and every other postwar president before would have done. But they also know now that what the US might do in the future will depend on who sits in the White House — and which party controls Congress — in 2025 and beyond.

In that context, it’s not surprising to hear serious discussion in countries that have highly adversarial neighbors, like South Korea or Poland, about obtaining nuclear weapons of their own.

Even though nuclear weapons are not a failsafe guarantee against any military action, as Iran’s recent missile strikes on the territory of nuclear-armed Pakistan demonstrate, “what nuclear weapons are good at doing is deterring existential threats or major wars,” said Panda.

For all that deterrence often fails, we shouldn’t overlook that since 1945, no country has used a nuclear weapon on the battlefield and only a small handful of new countries have obtained them, something that would likely have come as a surprise to leaders at the dawn of the nuclear era. But a future in which numerous countries feel so unsafe that they choose to obtain nuclear weapons is precisely the future that decades of postwar US security policy dedicated to nuclear nonproliferation sought to prevent. Averting that future is, as President John F. Kennedy said in a famous speech to the UN in 1963, “a practical matter of life or death.”

Whether it can still be averted may come down to whether other countries — both friends and enemies — still believe the US will make good on its word.