With nuclear modernization programs under way across a range of countries, Russia asserting its right to deploy nuclear weapons in the Crimea, NATO reviewing the role of nuclear weapons in the alliance, and a recent report in the U.S. arguing for a more versatile arsenal of tactical nuclear weapons, it’s clear the world’s revisiting an old problem: how to build effective nuclear deterrence arrangements.

Since the end of the Cold War, thinking about deterrence issues has been mainly confined to the academic and think-tank world. But policymakers are now having to re-engage with those issues. And the problem has a new twist: we no longer enjoy the luxury of a bipolar world. Indeed, as Therese Delpech observed in her RAND monograph Nuclear deterrence in the 21st century, nowadays “the actors are more diverse, more opaque, and sometimes more reckless.”

Done properly, deterrence is a contest in threats and nerve, or—to use Thomas Schelling’s phraseology—“the manipulation of risk.” (The chapter so titled in Schelling’s Arms and influence is a great starting point for anyone wanting to think through the broader deterrence problem.) That helps explain why some thought the concept ‘ugly’. It’s hard to make a policy threatening massive damage to societies and civilians sound noble and aspirational. Still, the bad news is that the alternatives are worse. And if deterrence is going to remain the dominant approach in nuclear weapon strategy, we need to fit the strategy to the contemporary geopolitical environment.

Historical experience of the deterrence problem is greatest in relation to two competing superpowers, separated by intercontinental distances, endowed with the resources to manage challenges, and both knowing well the costs of major war. We’ve had relatively little experience of nuclear deterrence in contests between giants and midgets (US v North Korea), between established and fast-rising powers (US v China), and amongst players in a multipolar system. Even our understanding of the role nuclear deterrence plays in relations between regional rivals (think South Asia) remains under-developed. It’s entirely possible that the old superpower deterrence model might not fit those new challenges well. Indeed, maybe the old model doesn’t even fit the US–Russian strategic relationship well these days: Russia’s no longer governed by a sclerotic CPSU.

Some years back INSS’ Elaine Bunn (now a senior official in the Obama administration) wrote a paper unpacking the notion of ‘tailored’ deterrence introduced in the 2006 Quadrennial Defense Review. True, deterrence has always been characterized by particular strategic wrinkles, but Bunn’s paper was an attempt to bring those wrinkles to the fore in relation to the possibility of a nuclear-armed North Korea, Iran, or transnational terrorist group. Her exploration of three different forms of tailoring—tailoring to specific actors and specific situations; tailoring capabilities; and tailoring communications—helps to illustrate the growing complexity of the deterrence challenge.

It now seems likely that we’re headed back into a set of complicated deterrence debates. A strategy that might make sense in one strategic setting—for example, a degree of restraint by a giant engaged in a conflict with a midget—might well risk flagging unintended messages in another. In the giant–midget case, almost any crossing of the nuclear threshold by the giant risks imposing a set of desperate choices on the midget’s leadership, and desperate choices tend not to be good ones.

Deterrence in the context of an established power versus a fast-rising power has a different wrinkle. One effect of a deterrence-dominated world is to reward passivity over initiative. As Schelling notes, in the world of the arthritic, passivity tends to be the default choice. But fast-rising powers aren’t arthritic. Turning one aside from a revisionist agenda will probably be more challenging than deterring another established player.

Multipolarity brings its own wrinkles, including a more mixed set of adversarial relationships, asymmetrical contests, inadvertent signalling, and third-party exploitation of bilateral rivalries. Capability issues become more vexed: actors require the capabilities to deter and defend against another, but also the residual capabilities to remain a player in other contests. The pressure must surely be towards larger rather than smaller arsenals. And reputational issues become more dominant: just as Margaret Thatcher fought the Falklands War in part to show the Soviet Union that the West wouldn’t buckle in the face of force, so too players in a multipolar nuclear world will want to show resolve in one contest because of its implications for others.

Finally, and perhaps most controversially, deterrence turns upon a credible threat to cross the nuclear threshold if push comes to shove. During the 1960s the U.S. advocated a doctrine of flexible response, arguing for a model of deterrence that would fail in small packets rather than in one catastrophic breakdown. Notwithstanding the giant–midget problem outlined above, there’s usually good sense behind such a doctrine: it makes deterrent threats more credible, avoids global annihilation in any initial crossing of the nuclear threshold, maintains a degree of ‘intra-war deterrence’ from the options still on the table, and optimizes prospects for negotiated war termination. But historically the doctrine invited questions about the relative balance between usability and credibility in US nuclear policy—questions buried rather than resolved by the end of the Cold War.

Tailoring, messaging, usability, credibility, and thresholds: I suspect policymakers will soon be thinking about all those questions again, across a range of deterrence relationships.

This piece first appeared in ASPI’s The Strategist here.