The decision to deploy nuclear-powered ballistic missile submarines (SSBNs) in the years to come will be a product of the major paradigms and concepts used to manage nuclear dangers more broadly. Recently, an emerging literature has pointed to a change in the way that at least the major powers plan to mitigate nuclear threats to their interests. This shift in thinking can be summarised as involving a greater reliance on strategic non-nuclear weapons—weapons and enabling systems that can be used to compromise an adversary’s nuclear forces using both kinetic and non-kinetic means that don’t involve nuclear weapons—and a decreased commitment to mutual vulnerability as the basis of strategic stability between nuclear-armed adversaries.

Strategic non-nuclear weapons include ballistic missile defence, conventional precision-strike missiles, anti-satellite weapons and anti-submarine weapons. When combined with advances in enabling platforms and systems such as elements of cyber, artificial intelligence and quantum technology, they can, in principle, be used to compromise an adversary’s nuclear capabilities, with serious implications for issues of deterrence and stability.

Traditional approaches to deterrence based on the threat of punishment now compete with policies based instead on deterrence by denial. Stability based on rational calculations under conditions of mutual vulnerability appears set to be even harder to maintain.

The potential for conventional counterforce strikes makes future scenarios involving ‘use them or lose them’ logic more likely for states that face adversaries armed with more sophisticated capabilities.

The current challenge to traditional nuclear deterrence relationships has a dual but paradoxical effect on the incentives to deploy sea-based nuclear weapons. In general, as missile silos (and even, over time, mobile land-based missiles), air fields, satellites, and command, control and communications stations become more vulnerable to counterforce attacks, the incentives to diversify a state’s nuclear force structure increase. In particular, SSBNs still remain the most secure form of second-strike capability, meaning that the further spread of strategic non-nuclear weapons is likely to result in ever more nuclear weapons being deployed at sea.

On the other hand, one of the key technologies that falls under the banner of strategic non-nuclear weapons is anti-submarine weapons themselves, and much analysis now is focusing on whether advances in this area may in fact undermine the perceived invulnerability of SSBNs. It’s important to note that growing concerns over the effects of new anti-submarine capabilities on strategic stability are, at least in part, based on projections about the future. Little serious analysis or commentary predicts that the oceans are going to become effectively transparent overnight. However, advances in sensing and signal processing in particular mean that it’s a serious possibility that the oceans will become significantly more transparent than they are today. And when it comes to nuclear force structure planning, serious possibilities are enough to keep decision-makers up at night.

As the development of strategic non-nuclear weapons and the associated shift in thinking about stable deterrence based on mutual vulnerability continues, policymakers and analysts will need to give serious attention to what might become the new determinants of stability in the global nuclear order.

The development of countermeasures will play an important role in mitigating the destabilising effects of disruptive technological breakthroughs in anti-submarine weaponry. The role of countermeasures is already evident in other domains. For example, as a reaction to US missile defence, both China and Russia today are placing increasing emphasis on hypersonic missiles because their combination of speed and manoeuvrability makes them extraordinarily difficult to defend against.

Countermeasures for anti-submarine weapons need not rely on kinetic effects. The development both of ever quieter SSBNs with smaller acoustic signatures and of new techniques of deception (for example, unmanned underwater vehicles designed to produce tonals that match those of SSBNs that are thought to have been identified by an adversary) can increase a state’s confidence that at least some of its SSBNs can remain undetected and uncompromised in a crisis.

Developments in anti-submarine weapons aimed at compromising SSBNs and developments in countermeasures aimed at mitigating those breakthroughs will take on a tit-for-tat dynamic in the years to come. This is not a new phenomenon, but as rapid increases in things such as sensing techniques and data processing allow for technological leaps in anti-submarine capabilities, countermeasures should be expected to take on a new and much greater importance.

Defensive measures for SSBNs aimed at increasing their reliability in the face of technological breakthroughs in anti-submarine weaponry are unlikely to solely rely on new technologies themselves. For example, James Holmes has suggested that both ‘bastion’ strategies for SSBNs (vessels constricted to a much smaller, actively defended area for patrols) and SSBNs being accompanied by convoys of ‘skirmisher’-type defensive units (adopting a similar principle to aircraft carrier battle groups) may be necessary to regain confidence in the survivability of SSBNs.

Stability needs to be seen as the most important goal and that will require a degree of what has been termed ‘security dilemma sensibility’ among the nuclear-armed powers. Leaders that develop security dilemma sensibility display an openness to the idea that, as Nicholas Wheeler has put it, ‘an adversary is acting out of fear and insecurity and not aggressive intent, as well as a recognition that one’s own actions have contributed to that fear’.

For example, future Chinese breakthroughs on quantum computing and their application to SSBN communication technology could be a positive development in the US–China strategic relationship. The more confidence Beijing has in the security of its second-strike capability, the less likely it is that a crisis between the US and China will inadvertently escalate.

Beyond unilateral measures, it may be possible, over the longer term, to negotiate, and design, limited multilateral efforts aimed at restoring stability between adversaries, including in relation to sea-based nuclear deployments. History suggests that confidence-building measures can play as important a role as formal arms control measures in reducing nuclear dangers, meaning that finding avenues for dialogue, even at a low level, should now be a top priority.

In the short term, the increasing salience of strategic non-nuclear weapons and the abandonment of deterrence strategies based on mutual vulnerability, is likely to continue to encourage states to deploy more SSBNs. Simultaneously, these forces will intensify the pressures to better protect SSBN fleets that are already deployed from technological breakthroughs in the anti-submarine weapons domain. Restraint in the deployment of anti-submarine capabilities may need to become a substitute for the more traditional tools used to instil stability in nuclear-armed relationships—restraint in defensive technology (such as missile defence) and negotiated limits on arms.

This piece was produced as part of the Indo-Pacific Strategy: Undersea Deterrence Project, undertaken by the ANU National Security College. This article is a shortened version of chapter 20, ‘Strategic non-nuclear weapons, SSBNs, and the new search for strategic stability, as published in the 2020 edited volume The future of the undersea deterrent: a global survey. Support for this project was provided by a grant from Carnegie Corporation of New York.