To commemorate the seventieth anniversary of the United States-South Korea alliance, President Yoon Suk-yeol of South Korea made a trip to the United States. As a result of the visit, the US and South Korean governments released the joint , which reaffirmed America’s commitment to extended deterrence in South Korea and reaffirmed South Korea’s commitment to the Nuclear Nonproliferation Treaty (NPT). The declaration also announced the creation of the Nuclear Consultative Group (NCG), which will offer South Korea a more substantive voice in nuclear planning. Moreover, the US confirmed that an Ohio-class ballistic missile submarine (SSBN) would make a planned port visit to South Korea.
The Trident D5 submarine-launched ballistic missiles (SLBMs) deployed aboard the SSBNs are said to have a range of over 10,000 km (6,200 miles). This means that an SSBN on patrol near California could strike targets located on the Korean Peninsula or even mainland China within 30 minutes. This is the fastest, more penetrable nuclear delivery option than either a B-52H flying over from the US or Guam armed with an AGM-86B air-launched cruise missile, or an F-35A hypothetically deployed in South Korea or Japan with a B61-12.
Risks to Forward-Deployed SSBNs
The SSBN's visit to South Korea, while an important political signal to reassure the South Korean public and demonstrate US resolve to nuclear-armed regional adversaries, has little direct military effectiveness. Rather, there are several potential risks.
First, the acoustic signatures of SSBNs are highly attractive data for potential adversaries. Each submarine has unique acoustic features. This sound signature is one of the most sensitive pieces of data about a submarine and a major asset for an adversary attempting to track it. When SSBNs—which typically do not deploy very far from the US mainland—appear nearby, North Korea and especially China have an ideal intelligence-gathering opportunity.
Second, port visits are the most physically vulnerable moment for SSBNs. Tracking and destroying US SSBNs is nearly impossible in the vastness of the Pacific Ocean. However, during a port visit, when the SSBN will be docking at a specific, visual, and public location, the survivability of the SSBN is significantly diminished. Given this vulnerability, SSBNs are likely to make port visits only during peacetime, and they are unlikely to visit ports of call during contingencies.
Third, it is not clear how often the SSBNs will visit South Korea. Of the 14 known SSBNs, only two to four are estimated to be on strategic deterrent patrols—one or two in the Pacific and one or two in the Atlantic. The others are under long-term maintenance, engaged in training, or at port. In short, unlike bombers, the scarcity of available SSBNs means there are only limited opportunities to demonstrate its visible presence in northeast Asia.
Opportunities for Forward Deployment to Enhance Future Extended Deterrence
However, there are ways to turn these potential risks and uncertainties into opportunities to enhance extended deterrence.
First, when the SSBN makes a port visit to South Korea, America’s northeast Asian allies should view this as a chance to improve interoperability with US forces to maximize the safety of the SSBN. Typically, when SSBNs go in or out of port, they are escorted by surface vessels. If there is a concern about adversary submarines trailing the SSBN, the surface vessels and attack submarines will have done delousing procedures to determine if an opposing submarine is nearby attempting to gather intelligence. As an example, South Korean and Japanese forces, in cooperation with the US, need to be on the lookout for Chinese, rather than North Korean, attack submarines, patrol aircraft, and acoustic sensors. In some cases, US or allied vessels may need to deploy countermeasures such as denial or disruption. The three countries should discuss, coordinate, plan, and share their capabilities to implement measures to enhance the safety of SSBNs during peacetime.
Second, in the medium to long term, the US and its allies need new theater nuclear options to fill the deterrence gap. SSBNs are inherently survivable but are less visible and flexible than bombers or dual-capable aircrafts (DCAs). Strategic bombers and DCAs are visible and flexible, but slow and less able to stay in the same airspace for long periods. In times of crisis, landing these aircraft at forward bases makes them vulnerable to Chinese and North Korean precision theater-strike capabilities. Ground-based nuclear missiles like the Pershing 2, if deployed, risk becoming a priority target for precision nuclear strikes in times of crisis.
The US needs to develop a survivable, low-yield, theater nuclear option that is more flexible than SSBNs and can remain in the same area for longer periods than bombers or DCA. This should have been the role of nuclear-armed sea-launched cruise missiles (SLCM-Ns). The Biden administration canceled this program, but it should be considered for redevelopment. However, even if the next administration were to authorize the development of SLCM-Ns, deploying them would take nearly a decade.
If redevelopment of SLCM-Ns as a more quickly feasible, underwater-based, theater nuclear option does not happen in the foreseeable future, then the US should consider developing the intermediate-range nuclear prompt strike (IR-NPS). The intermediate-range conventional prompt strike (IR-CPS) is scheduled to be integrated with Virginia-class nuclear-powered fast attack submarines (SSN) in 2028 to achieve full operational capability. As a first effort, the US Navy and the national laboratories should conduct a technology demonstration to modify and load existing or developing nuclear warheads on the 34.5-inch, two-stage rocket motors designed for the IR-CPS. Initial deployment of the LRHW (long-range hypersonic weapon), which shares most of the IR-NPS’s design, is coming this fall, and its rocket motors can begin initial production.
If the priority is to accelerate deployment, there is no need to put a nuclear warhead on the common-hypersonic glide body (C-HGB). A combination of a traditionally designed nuclear warhead and a rocket motor can be operated as a submarine-launched medium/intermediate-range ballistic missile (SLMRBM/SLIRBM) in an initial phase. And once hypersonic glide technology has matured, a phased upgrade to integrate nuclear warheads with the C-HGB would maintain a maneuverable theater nuclear option that could penetrate Russian and Chinese missile defenses, even if their systems become comparable to those of the US and Japan in the future. With New START largely invalidated by Russia's suspension of implementation, there are no arms control agreements to make deployment of the IR-NPS difficult, and by the time the IR-NPS is ready for deployment, New START will have expired anyway.
If Japan, South Korea, or other regional allies like Australia work together to support US theater nuclear deterrence, this cooperation would be an Indo-Pacific version of SNOWCAT (Support of Nuclear Operations With Conventional “Allied” Tactics), and the combination of IR-NPS and SSNs would fill the gap in the Indo-Pacific region's lack of theater nuclear forces.