Concepts are long-lived in the world of strategy—so long-lived that we need to revisit them periodically to confirm that their meaning hasn’t shifted. Lately, I’ve started thinking that the notion of a ‘second nuclear age’ has matured a lot during the last twenty years. Indeed, the concept has evolved through three distinct variations, each a little more worrying than its predecessor.

In its first formulation, the concept warned of the potential failure of deterrence doctrine when nuclear weapons spread to “rogue states” such as North Korea. That’s because strategists in the 1990s found it difficult to imagine the circumstances in which nuclear weapons would once more have the prominence in great-power relationships that they had during the Cold War years. In consequence, there was an emphasis placed on the new, the weak and the poor— “underdogs” Robert O’Neill once called them—as the future problems of the nuclear world.

In that vein, Keith Payne’s Deterrence in the Second Nuclear Age (1995) and Paul Bracken’s Fire in the East (1999) both signalled the difficulties that deterrence encountered from proliferation. Bracken wrote of a second nuclear age characterized by nationalism rather than ideology; a willingness to use other kinds of weapons of mass destruction, like chemical weapons; impoverished nuclear weapon states; shaky command and control systems; difficulties in communicating and bargaining with the West; deliberate reductions in conventional capabilities to permit greater nuclear capacities; and less willingness to model deterrence policies upon the strict logic of game theory.

Then, in 2004, almost a decade after writers initially began to contemplate the strategic significance of rogue nuclear powers, a small group of strategists—Kurt Campbell, Robert Einhorn and Mitchell Reiss—wrote of the emergence of a potential nuclear tipping point. Regional proliferators risked exciting small proliferation chains—and among status quo powers, not merely rogues. That book contained a set of case studies outlining possible proliferation by Egypt, Saudi Arabia, Syria, Turkey, Germany, Japan, South Korea and Taiwan. The broader message about the second nuclear age became more complicated. Rogues were bad enough, but proliferation chains might, indeed, undo the broader global nuclear order, set at its core by the Nuclear Nonproliferation Treaty (NPT). The NPT had obliged most states to choose their future nuclear identity at the end of the 1960s and beginning of the 1970s, and thirty-odd years down the track, there was the chance that some had repented of their choice.

In 2015, I think we’re beginning to see the second nuclear age in its third variation. Paul Bracken warned in his 2012 work The Second Nuclear Age that nuclear weapons were returning to relevance among the traditional great-power members of the nuclear club, but tensions between those players have increased noticeably since then. The possibility that seemed remote in the 1990s now seems less remote. The P5 are modernizing their weapons—and it’s strategy and not mere technological obsolescence that’s driving those modernization programs. In short, the strategic significance of nuclear weapons is going up in relation to the “top dogs,” and not merely in relation to the underdogs and their status-quo regional neighbors.

This third variation of the second nuclear age (an ungainly expression) carries us into even more difficult terrain. Tensions between Russia and the West have increased, bringing with them both Russian behavior—like long-range bomber patrols—reminiscent of the Cold War years and echoes of the nuclear debates in Europe during that period. In Asia, uncertainties resulting from the growth of Chinese conventional power are driving a brisker discussion about US extended nuclear assurance. In the Middle East, Iran’s nuclear future—and thus the region’s—is murky.

Nuclear weapons are making a comeback, but we’re sorely lacking in a good understanding of where they’re going to fit in both national and international strategies. If we can’t get rid of them—and we can’t—how can they be leveraged in the current international environment to provide the greatest contribution to international security? Since the end of the Cold War, the generation of nuclear strategic thinkers who concentrated upon such questions has largely passed. A new generation needs to address the topic.

Meanwhile, the topic itself has become harder. Future nuclear strategists aren’t just dealing with the Cold War problem of how to ensure deterrence and stability in a bipolar relationship of risk-averse and economically-decoupled superpowers. Great-power strategic relationships are more multipolar. There are larger worries about the possible leakage of nuclear weapons to non-state actors. And those two earlier waves of the second nuclear age did reflect genuine concerns: rogues and potential proliferation chains now haunt the order in a way they haven’t since the end of the 1960s. A major challenge lies ahead in a field in which we’ve been shedding expertize rather than nurturing it.