Given North Korea’s continued development of a submarine launched ballistic missile (SLBM), South Korea may be closer than ever to securing the technical support it would require from the United States to develop its own nuclear-powered submarines (SSNs). Indeed, South Korean President Moon Jae-in and U.S. President Donald Trump may have discussed South Korea’s longstanding ambitions to develop SSNs during Trump’s trip to the UN last week.
While South Korean defense circles have discussed the potential costs and benefits of South Korean SSNs, there has been little such discussion in the United States. It is critical that the U.S. defense-policy community seriously consider the potential consequences of this capability for the U.S.-South Korea alliance. Below, we highlight some of the potential risks and rewards of a U.S.-assisted South Korean SSN program for allied interests.
One benefit of South Korean SSNs would be to strengthen the allies’ ability to carry out the “4D” operational concept . This concept emphasizes the need to “detect, disrupt, defend against, and destroy” North Korean nuclear missiles. The so-called “Kill Chain” system, a set of sensors and deep strike capabilities that would allow the allies to preemptively eliminate a North Korean missile prior to launch, forms a key part of the 4D concept. North Korea’s progress toward a SLBM, however, has cast doubt on the sufficiency of the current Kill Chain system. Unless the allies enhance their joint anti-submarine warfare (ASW) capabilities, a North Korean missile sub could render 4D far more challenging.
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This is where South Korean SSNs could play a decisive role. While SSNs are noisier and bulkier than South Korea’s Son Won-Il- and Chang Bogo-class diesel electric submarines (SSKs), they offer the advantage of heightened endurance, stronger power output, more capable sensors and faster underwater speeds. As the U.S. Navy has highlighted , SSNs can also deploy and recharge battery-powered unmanned underwater vehicles (UUVs) for reconnaissance purposes.
SSNs could therefore bolster both the “detect” and “destroy” functions of the 4D operational concept. They could supplement seabed sensors, P-3 Orion aircraft, and other allied ASW capabilities by acting as stealthy, forward-deployed sentries in North Korean littorals. In this capacity, they could monitor North Korean ports and trail missile subs with the help of UUVs and advanced sensors, without the need to reveal their positions to recharge their batteries like SSKs. And, if need be, SSNs’ superior speed and armaments could be utilized to preventively eliminate North Korean missile subs, providing the basis an “underwater Kill Chain.”
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But why not simply have the United States regularly deploy its SSNs to the East and West Seas to fulfill this task? Unfortunately, the U.S. SSN fleet has struggled to keep pace with mounting challenges around the world including a more assertive China and resurgent Russia. The additional burden of providing an underwater Kill Chain would only further strain the U.S. fleet. As such, if the South Koreans can field a small SSN fleet capable of addressing the 4D requirements in the Korean littorals, it would serve to augment rather than simply duplicate U.S. capabilities.
Coercion and Deterrence
A South Korean SSN could also strengthen the allies’ efforts ability to engage in coercive bargaining with North Korea. By improving the allies’ ability to carry out the 4D operational concept, South Korean SSNs should enhance “deterrence by denial” against North Korean nuclear threats. In doing so, SSNs would not only reduce the chances that the North would seek to use its SLBM capabilities, they also could curtail the risk that the North will be emboldened by SLBMs. In other words, if allied 4D capabilities are robust and effective, the North is less likely to believe that it's SLBMs will allow it to carry out more aggressive provocations against the allies without fear of retaliation.
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The transfer of technology by the United States and costly investment of resources by South Korea could function as a costly signal of allied resolve. By bearing the additional costs of upgrading this capability, both allies would further highlight their determination to resist North Korean revisionism and reduce the risk of deterrence failure.
A U.S.-supported effort by South Korea to develop SSNs could also have beneficial consequences for alliance cohesion. One of the recurring sources of tension in the alliance is South Korea’s fear of abandonment . These concerns will become all the more acute as the North moves toward the ability to strike the mainland United States (many in South Korea worry that the United States would not defend Seoul if it involves the risk of a North Korean nuclear strike on a major American city). U.S. assistance with naval nuclear propulsion, however, would serve as a highly visible and costly signal of American commitment to its ally’s defense, bolstering South Korea’s confidence in the alliance’s dependability.