Current and aspiring nuclear great powers (the United States, Russia, and China), together with other comparatively small nuclear weapons states (either declared or widely acknowledged as such), are investing in expensive and expansive modernization of their nuclear arsenals. This pattern of growing commitment to larger and costlier nuclear weapons deployments is predicated on the assumption that nuclear weapons are a necessary and sufficient deterrent to a major war, including nuclear war. But that assumption is now under widespread challenge.

What we are seeing is a growing willingness of state and non-state actors to engage in large-scale conventional and unconventional warfare, even against the interests of nuclear powers. It turns out that, without the capability to deter or win conventional wars or unconventional attacks against vital interests, a state’s nuclear arsenal is, effectively, a one-dimensional success story sitting atop a glue factory of military insufficiency.

Dissenters of the preceding view might argue that nuclear weapons serve to deter a nuclear attack against the state and its vital interests and nuclear blackmail by one state against another or its allies. This concept is of little consolation to practical heads of state and military planners. A deliberate nuclear strike “out of the blue” by one nuclear power against another, not preceded by a conventional war, is one of the least likely paths to nuclear war. More likely is the expansion of a conventional war into a decision by one side to engage in nuclear first use. 

The side that is winning the conventional war is less likely to engage in nuclear first use or first strike than the losing side. Resorting to nuclear first use would probably be a decision to rescue a losing position in a conventional war. Still, against a nuclear-armed opponent, nuclear second use in retaliation could not be ruled out—indeed, it would almost certainly be expected. Thus, a power that decided on a first-use nuclear policy has wittingly opened the door to a process of escalation over which further control requires a two-sided tacit agreement not to climb the ladder any higher.

Russia’s war against Ukraine beginning in February 2022 is an example of conventional deterrence failure that, as the fighting grows in terms of economic costs and societal destruction, invites an eventual expansion to nuclear war. NATO’s considerable support for Ukraine has kept the latter in the fight, together with the tenacity of Ukrainian resistance, the ingenuity of its intelligence services and tactical commanders in force employment, and the inability of Russia to close the deal with its own conventional forces and military assets greatly outnumbering those of Ukraine. 

This stalemate has led to Russian frustration that expresses itself in periodic threats of nuclear first use by Russian President Vladimir Putin, other members of the Russian government, and noted Russian academics. Just as NATO failed in conventional deterrence before the outbreak of war in February 2022, Russia has failed to compel extensive Ukrainian resistance, including strikes into Russian territory with drones and long-range missiles supplied by the United States and NATO allies. Sooner or later, the military stalemate in Ukraine will have to give way to negotiations and a peace agreement—however distasteful such a deal might be to hardliners on all sides.

Further evidence of the apparent futility or declining relevance of nuclear deterrence, when it is unsupported by conventional deterrence based on war-winning capability, is provided by ongoing wars in the Middle East. Israel is a nuclear weapons state, yet it was attacked on October 7, 2023, by Hamas with the support of Iran because Israeli conventional deterrence failed (together with strategic intelligence). In addition, Iran aspires to join the club of nuclear weapons states, and neither the United States nor Israel has been able to deter Tehran’s quest for nukes by posing a credible conventional threat.

A nuclear Iran opens the door to a Middle Eastern proliferation nightmare that could include Saudi Arabia, Egypt, and Turkey as aspiring nuclear weapons states. However, unlike the U.S. attack on Saddam Hussein’s Iraq in 2003, no coup de main against the regime in Tehran appears feasible. Economic sanctions and cyberwar are about all that remains in the U.S. tool kit for dealing with the ayatollahs and their supporting clique in the Islamic Revolutionary Guard Corps.

Speaking of embarrassing futilities in conventional deterrence—how is it that a group of terrorists holed up in Yemen, who were once limited to small arms and jeeps, have assembled an armory of missiles and drones to all but shut down traffic in the Red Sea, holding hostage much of the supply chain that affects countries in all regions? Great powers used to call people like this “pirates” and dispatched sufficient forces to treat them accordingly—as outlaws, not governments. Yet the Houthis, supported by Iran, continue to plague mariners of many nationalities without fear of retaliation from all but a few countries. The United States and the United Kingdom have attempted to push back and strike against identifiable terrorist weapons caches and command strongholds. Still, frankly, the situation is a global embarrassment for the international community, including the nuclear powers.

Some analysts may object that it is a stretch to discuss nuclear deterrence with respect to the situation in Yemen. However, apart from deterrence, one assumed characteristic of nuclear weapons is that they allow their possessors a certain amount of swaggering and additional reputation that they are not to be messed with. But this reputational overhang from nuclear possession looms less important in dealing with prospective conventional warriors and unconventional attackers than does the ability to disarm your opponent or to displace his regime and, in the case of terrorists, strike them with precision and lethality. America’s nuclear deterrent is not in doubt per se, but the United States and other leading military powers are having a harder time making conventional deterrence stick. In turn, unreliable conventional deterrence invites leaders to substitute military-strategic bluster or, if available, reach for nuclear coercive bandaids. 

An example of the latter is North Korea. Pyongyang would almost certainly lose a major conventional war against South Korea, which the United States and other allies support. As a result, North Korea has ramped up its nuclear-strategic capabilities and even tested intercontinental ballistic missiles (ICBMs) capable of reaching U.S. national territory, not to mention numerous missiles for attacks against American allies in the Indo-Pacific. This dangerous nuclear posturing by North Korea could stimulate responsive nuclear weaponization by Japan, South Korea, and other regional actors who feel directly or indirectly threatened. 

Obviously, there are sensible counterarguments to the preceding example. One might contend, for example, that North Korea has survived as an outlaw regime, but Saddam Hussein in Iraq and Muammar Qaddafi in Libya did not because the Hussein and Qaddafi regimes lacked a nuclear deterrent to conventional aggression and regime change. From this perspective, even a small nuclear force can deter a more powerful conventional attacker by threatening to impose unacceptable costs on the latter. 

But this argument misses a critical point. In order to act as a credible deterrent, a nuclear force must be able to survive a first strike from putative attackers. If a small nuclear force can be destroyed preemptively with either conventional or nuclear weapons, it simply invites an attack on itself. Nonsurvivable nuclear forces are instigators of deterrence failure instead of reliable insurance policies against aggression. 

The most stressful potential shock test for U.S. conventional deterrence in the near term is the challenge of two simultaneous or nearly simultaneous large-scale conflicts in Europe and Asia. The mature Cold War standard for U.S. force planning for two major regional conflicts or major regional contingencies lapsed after the end of the Cold War and the demise of the Soviet Union. The United States and its allies in Europe and Asia must now expect more assertive political and military behavior from Russia and China than hitherto, and the two powers have already increased their cooperation through joint military exercises. 

This cooperation does not mean that their partnership will extend to combined military operations in the near future, and cooperation is far short of the military interoperability needed for shared responsibility in battle. In addition, the United States has allied support in Europe and Asia that is more than symbolic and capable of providing technology, training, and forces to support deterrence and defense in theater. Nevertheless, deterrence and defense requirements for this most demanding case will require policymakers with strategic vision, commanders with imagination and daring, and durable linkages among allied partners across the conflict domains of land, sea, air, space, and cyberspace. Unfortunately, conventional deterrence and defense failure in this scenario can open the door to nuclear escalation with unforeseeable consequences.

Stephen J. Cimbala is a distinguished professor of political science at Penn State Brandywine and the author of numerous books and articles on international security studies, defense policy, nuclear weapons and arms control, intelligence, and other fields. View a listing of Dr. Cimbala’s authored books, book chapters, and journal articles here.