In his introduction to this Interpreter debate, Rory Medcalf raises the important question of how nuclear-armed ballistic-missile submarine (SSBN) programs in Asia influence strategic stability.

Making such an assessment for any one weapons system in isolation is fraught with difficulty, as judgments are inevitably based on assumptions about doctrine, employment, escalation, and strategic concepts. Technical details also matter a lot, as was demonstrated in the late Cold War by the development of the highly precise Trident D5 (which gave the US SSBN fleet the ability to conduct counterforce missions against hardened targets) on the one hand, and the appearance of very quiet Soviet submarines (pictured) on the other. Strategic stability is thus always a question of net assessment.

Viewed in this light, the scale and scope of the programs under development in Asia today seem unlikely to change fundamental power relationships and military balances. India and China have toyed with SSBN technology for decades, and it is difficult to see either action-reaction patterns, or an out-of-character acceleration, that would indicate an incipient SSBN arms race. That said, the fact that SSBNs are now being introduced into the regional mix of capabilities throws a useful spotlight on the influence of geography, and on Chinese views about the vulnerability of their nuclear forces. Both of these factors are of fundamental importance to net-assessment-based judgments of strategic stability in the Western Pacific, and highlight the strengths of the current strategic order that China must still overcome.

The attraction of SSBNs is that they can be difficult to find and destroy, particularly if they are either isolated from adversary ASW forces in a 'bastion' or able to hide in the vast expanses of deep water found in the Pacific, Atlantic, and under the Arctic ice cap. Strategic geography thus favoured the employment of SSBNs by the main nuclear powers of the Cold War (the US, Soviet Union, France and Britain), whose submarine bases had direct access to suitable deployment areas.

Not so in China's case. While the waters of the northern South China Sea are deep, they are also confined.

The need to pass through chokepoints into the Pacific places Chinese submarines at a disadvantage, as it makes it easier to for US and allied ASW forces to detect and track Chinese SSBN patrols passing into the Pacific, or to block them through mining in wartime. Moreover, the South China Sea is ringed by US allies, and any 'bastion' the PLA Navy might attempt to establish could be contested by a range of US and allied systems operating from friendly territory. Those systems would of course themselves be at risk of Chinese attack, but the heart of a conventional battle would not be a good place for an SSBN to be.

'Deploying' the SSBN in the cave complex on Hainan Island might align more closely with the Chinese preference to keep close control of nuclear warheads, but it would significantly limit the strategic benefit of having an SSBN capability: if they remain inside the caves too long, the Chinese SSBN force risks being disabled by a US strike (even if this required nuclear weapons). Should they leave the caves in a crisis, however, China risks sending inadvertent escalatory signals, and the boats would enter waters likely to be teeming with US attack submarines.

All of this raises the question of why China is developing SSBNs in the first place. The reasons are far from clear, and a coherent strategic rationale may not even exist in Chinese minds. SSBN development is consistent with the long-term features of Chinese nuclear strategy and force modernisation, which emphasise survivability through dispersion of its land-based systems. With the DF-31 and DF-31A, China can now retire its only silo-based system, the large DF-5 ICBM, and rely completely on solid-fuel, road mobile nuclear missiles.

And yet the development of an SSBN force indicates that China is not fully confident in the survivability of its land-based nuclear forces, and is at least hedging its bets. This is not fully unjustified, as land-based missiles still have to operate in prepared deployment areas that are vulnerable to the large-scale nuclear suppression campaign, of which the US is still capable. US attempts to build a survivable launcher for its own land-mobile Midgetman missiles during the Cold War are a salutary reminder of the inherent fragility of mobile ICBMs to nuclear blast pressure. Adding SSBNs to the mix would certainly complicate US planning for the unlikely event that it should ever consider a disarming strike on China, but it does not change the basic source of China's vulnerability, which lies in the numerical imbalance of the nuclear forces of both countries.

Investment in SSBNs thus indicates that China remains uncomfortable with the limits imposed by its small nuclear arsenal. But the scale of China's investment doesn't really address the issue. SSBNs put a lot of eggs into one basket and are no panacea for the problem of survivability, especially given China's geographic disadvantage. As such, their appearance in the region is a useful reminder that the broader factors underpinning strategic stability in the region — US military capabilities, including its nuclear arsenal, and the alliances they support in the Asian littoral — have a lot going for them still.

The Lowy Institute gratefully acknowledges support from the John D and Catherine T MacArthur Foundation for this Interpreter debate, which is part of a broader research, dialogue and outreach project on strategic stability in Indo-Pacific Asia