Ballistic missile submarines (SSBNs) became the seagoing platform of choice for the deployment of strategic nuclear weapons by 1960, with the availability of submarine-launched ballistic missiles. Today there are five countries with operational SSBNs. The US, the UK, France and Russia all have a major part of their deterrent capability deployed on SSBNs, while China has three or four SSBNs, though not all are operationally available.

Before getting into a discussion on how the spread of sea-based nuclear weapons in Asia would affect stability, it is worth revisiting some concepts regarding 'stability'. To clarify, this discussion is about nuclear stability, not about prevention of conventional conflict. In other words, our frame of reference is to examine whether the possession and deployment of nuclear-armed submarines in Asia would provoke, or conversely prevent, escalation of a conflict over the nuclear threshold. With this in mind, the following is a brief review of the existing concepts:

  1. Strategic stability exists when there is mutual acceptance of relative nuclear force levels (which is not the same thing as the forces being equal) and neither side has the intent or desire to alter it. A change in this relativity could induce willingness to strike first, either by the side that perceives itself as weaker (as a pre-emptive defensive measure) or by the side that sees itself as stronger and therefore immune from effective counterstrike.
  2. Crisis stability exists when the nuclear weapons of both sides are not vulnerable to inadvertent or unauthorised launch, when both sides are confident in each other's determination to avoid escalation, and when the command and control (both politico-military as well as technical) is robust and can withstand political crises.
  3. Deterrence stability prevails when each side knows that the nuclear forces of the other will survive an attack and be able to deliver an effective retaliatory strike with consequences that will be unacceptable to it. 

The only Asian country that currently has SSBNs in service is China, though their detailed operational status is not clear. India is in the process of operationalising its first SSBN; however it is not clear when a matching submarine-launched ballistic missile (SLBM) will be available. Pakistan does not have any SSBNs and has not declared any intention of building or acquiring any, though there are reports of it considering nuclear warheads for its submarine-launched cruise missiles. The only other nuclear weapons deployed in the Asian maritime region are those on US and Russian submarines. With this background we may consider the question at issue.

Let us for convenience consider the relevant countries in dyads, beginning with the US-China equation. The US has overwhelming nuclear superiority in forces as well as technology, and the SSBNs that China can deploy against it will not seriously affect the balance between them for some time to come. If however China's SSBN program goes smoothly and it has five or six operational units in the next three or four years, that would be a significant challenge which the US would have to address. From a theoretical standpoint it is more destabilising when one nuclear power in a dyad has the overwhelming advantage, and China having a demonstrable ability to retaliate, albeit with a much smaller force, will reduce the possibility of a preemptive strike by an adversary. 

A similar situation exists with the India-China dyad. China's nuclear superiority rules out any first strike attempt by India. With a far smaller arsenal than China's, India is vulnerable to a first strike. The deployment of SLBMs will strengthen India's second-strike capability, on which it is largely dependent because of its 'no first use' policy. India is not pursuing parity in nuclear forces with China; if it did that would create arms race instability. The Indian SSBN program will not address the nuclear imbalance with China, but will improve strategic stability by giving India a credible ability to retaliate to a nuclear attack. 

The India-Pakistan equation is more fraught. Pakistan's nuclear arsenal is a little larger than India's and growing rapidly, according to the Bulletin of Atomic Scientists. According to reports, Pakistan also has a nuclear submarine building program, and although Pakistan has no known plan to deploy ballistic missiles at sea, there have been reports of plans to miniaturise a warhead for fitment on submarine-launched cruise missiles. Considering India and Pakistan share a long land border, and potential targets on both sides are a few minutes' flying time apart, it is hard to see how the Indian SSBN, when operational, would in any significant way change the nuclear stability equation. On the other hand, just as a destabilising element has been created on land by the introduction of Pakistan's Nasr tactical nuclear missile, if Pakistan does fit submarine-launched cruise missiles with nuclear warheads it would result in the delegation of the control of nuclear weapons to the tactical level and thus dangerously destabilise the situation.

To conclude: the deployment of sea-based nuclear weapons will not destabilise the nuclear balance in the Indo-Pacific provided this is restricted to ballistic weapons with centralised launch authority and the requisite command and control structures. The situation would, however, become extremely unstable by the arming of ships and submarines with nuclear-tipped cruise missiles.

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.