The US undersea deterrent is the most survivable leg of America’s nuclear triad of ground-, air- and sea-based nuclear capabilities. The sea-based leg, however, is also the most brittle of the three. If an alert nuclear-powered ballistic missile submarine (SSBN) is prevented from launching its missiles, is unable to communicate with commanders ashore, or is destroyed, all of its missiles become unavailable at once. If only one SSBN is on alert patrol, this could eliminate an entire leg of the triad.

With the return of great-power competition between the United States, China and Russia, the importance of the undersea deterrent has led to increased efforts by US adversaries to develop new ways to find and hold at-risk nuclear submarines.

Should improvements to anti-submarine warfare (ASW) capabilities render future US SSBNs vulnerable, the strategic risks may be increasingly borne by allies in the Indo-Pacific that rely on US extended deterrence.

Although other nuclear states maintain arsenals primarily to deter attacks against their homelands, the US nuclear arsenal is also designed to extend US deterrence and defend America’s allies in Europe and the Indo-Pacific from both nuclear and conventional security threats.

Due to extended deterrence, alliance politics have been more embedded in US nuclear decision-making than in that of any other nuclear state, both during the Cold War and in the current era. Consequently, the provision of extended deterrence requires the US to maintain a level of transparency about the size, scope and intended use of its nuclear arsenal that’s not required of either China or Russia.

The Trump administration’s nuclear posture review, released in early 2018, takes a mainstream position on the role and use of nuclear weapons as part of the US national strategy, and its continuities outweigh its departures from the strategic direction and policy of previous administrations. It also reflects a return to a traditional bipartisan consensus on the value of America’s nuclear arsenal by removing objectives for the eventual elimination of both US and global nuclear weapons.

What does this policy mean for undersea deterrence and the future US SSBN fleet?

The US is recapitalising its undersea deterrent by replacing its fleet of 14 ageing Ohio-class SSBNs with 12 Columbia-class SSBNs in cooperation with the UK Successor-class SSBN program. The two countries’ SSBNs will use a common missile compartment, similar fire control systems and the same Trident II submarine-launched ballistic missiles. If any of these components experience delays or technical failures, both programs will incur the associated risks.

In the 2030s, only one of the 12 US Columbia-class SSBNs will likely be on alert patrol at a time in the Atlantic and Pacific oceans, with between one and two at sea as a backup. The brittleness of the undersea deterrent and its role as the survivable US second-strike option provide incentives for adversaries to develop ways to hold it at risk or suppress its effective operation.

The projected Columbia-class cost of US$6–7 billion per boat is more than twice that of the Ohio-class SSBN, when adjusted for inflation. It is also about one-third to one-quarter of the US Navy’s annual shipbuilding budget.

To increase the funding available for other programs and constrain defence spending, leaders in the US Congress argue the military should reconsider its plans for recapitalising the US nuclear deterrent, including reducing the number of SSBNs. Further reductions in the SSBN fleet, however, would lower the number of backup submarines at sea, increase the brittleness of the force, and further encourage adversaries to develop new ASW capabilities that could hold SSBNs at risk.

Most submarine designs, including that of the Ohio-class, are optimised for acoustic quieting to reduce their vulnerability to passive sonar, the predominant type of ASW sensor. The Columbia class will continue this focus, and will incorporate electric propulsion at great expense to further reduce its signature. New ASW technologies, however, are reducing the reliance on noise generated by a target submarine. These technologies include low-frequency active sonars, submarine wake detection and sonar sensors.

Detection of an SSBN doesn’t need to result in a successful attack for the submarine to be neutralised. The inherent limitations of submarines—lack of self-defence and slow speed—require an SSBN to evade even ineffective attacks, and if it continues to be pursued it may be unable to establish conditions for launch, which normally require slow speed and shallow depth.

An adversary could exploit these limitations by using a network of active sonars and simple, inexpensive torpedoes or depth bombs to find and suppress possible SSBNs over an area of hundreds of square miles. The range of the Trident enables SSBNs to patrol areas covering thousands of square miles, but they would still be vulnerable to detection when they left and returned to their US home bases.

New ASW threats would have a greater impact on the UK and France, which only have an undersea nuclear deterrent. They would likely increase their reliance on US extended deterrence, which would compound the risk created by new threats to SSBNs. These risks are mitigated by the US nuclear triad, which the US government is modernising at a cost of more than US$300 billion during the next two decades.

The cost of sustaining a nuclear triad is a concern for US leaders, but the risks created by moving to a dyad or a single undersea leg wouldn’t accrue only to the United States. Allies such as South Korea, Japan and Australia that depend on US security assurances would be affected as well. As a result, future vulnerabilities to SSBNs that could arise due to the improving Chinese navy may convey more risk to allies than to the United States.

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 6, ‘The US sea-based deterrent in a new era’, 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.