SLCM’s Role in Deterring Regional Adversaries

America’s nuclear triad will soon face a significant credibility problem from the burgeoning nuclear arsenals of regional states. While still years away, a number of regional states will eventually possess the ability to field a reliable intercontinental ballistic missile (ICBM) topped by a nuclear warhead.¹ North Korea’s recent test launches of the Hwasong-14 and Hwasong-15 missiles illustrates the potential beginning of an era where regional states possess the capability to potentially threaten the continental United States with nuclear weapons. The 2018 U.S. Nuclear Posture Review (NPR) reaffirms this assessment, stating “North Korea may now be only months away from the capability to strike the United States with nuclear-armed ballistic missiles.”²

Any state armed with several dozen ICBMs theoretically has the capability to target American bomber bases. While it potentially will be decades before any regional state possesses this capability, if a regional state launched a successful first strike against the bomber leg of the nuclear triad, U.S. leaders would possess few credible response options. The most likely U.S. options in the instance of a successful first strike against the bombers would be a retaliatory strike with ICBMs or submarine launched ballistic missiles (SLBMs). However, U.S. leaders may reasonably fear that larger peer competitors could potentially mistake the trajectory of an ICBM or SLBM headed toward a regional adversary as intended for their own countries.³ U.S. leaders would also want to avoid the significant radiation associated with an ICBM or SLBM attack from drifting over neighboring states after a U.S. nuclear response.⁴ A theoretical first strike by a regional adversary might require a nonstrategic nuclear response to avoid the aforementioned concerns. This type of response has traditionally only been delivered via the bomber leg of the triad.⁵ Therefore, once the U.S. bomber force is vulnerable to a first strike by a regional adversary, the United States will find it increasingly difficult to deter that state.

The most probable solution to this impending strategic dilemma would be to develop a nuclear-tipped sea launched cruise missile (SLCM). The SLCM achieves strategic stability with an ICBM equipped regional state in several key ways. In the above scenario, the only low-yield-capable leg of the triad, the bomber force, is inherently vulnerable to destruction from the ICBMs of a regional adversary.⁶ However, a small force of SLCM equipped destroyers or submarines would balance this threat to U.S. nonstrategic nuclear weapons by their ability to remain at sea.⁷ This force would position itself out of the reach of regional navies, and also remain mobile and stealthy enough to prevent successful targeting by an adversary state’s ballistic missile force. The SLCM therefore provides a survivable nonstrategic nuclear capability to respond to nuclear first use by a regional state.

In addition, the trajectory of retaliatory ICBM or SLBM launches intended against a regional adversary might initially be mistaken as directed against larger peer competitors or nearby neutral states. This risks an unintentional widening of the conflict, weakening the credibility of using either of these capabilities to respond to a nuclear attack from a regional state.⁸ The SLCM mitigates these concerns, as it would be possible to launch the missile from a location at sea where its trajectory would unambiguously point towards the regional aggressor. This would lessen the potential for other states in the region to mistake the intended target of a U.S. retaliatory response. The SLCM will also likely be equipped with a smaller warhead than found on ICBMs or SLBMs, which lowers the risk to nearby U.S. allies from nuclear fallout. The 2018 NPR provides evidence for this, stating “SLCM will provide a needed non-strategic regional presence,” suggesting that it will carry a low-yield warhead.⁹

Opponents of SLCM argue that “it is not apparent what regional security problem a nuclear-armed SLCM would solve.”¹⁰ Granted, the regional security problem illustrated above will take years to develop. Deploying a SLCM capability is not intended to solve any current regional security problem. Instead, its development should be designed to mitigate highly probable regional security dynamics in the near future. The SLCM should be viewed as a hedge against capabilities that may at some point threaten the credibility of the U.S. nuclear deterrent.

Other opponents state that the SLCM is redundant, since the current fleet of U.S. dual capable aircraft (DCA) armed with the low-yield B-61 warhead already provide a nonstrategic nuclear deterrent.¹¹ This argument misses two key details in the context of deterring a first strike by a regional state. First, it is public knowledge that the United States no longer stores nuclear weapons in every corner of the globe, as evidenced by its removal of nuclear weapons from South Korea in 1991 and in Japan in 1972.¹² In the event of a first strike by a regional state, this would likely require the United States to transfer nuclear weapons for DCA use to bases near the regional adversary. This process would likely be prohibitively time consuming, especially if the U.S. feared additional strikes. Since all DCA capable bases are also likely within range of a regional adversary’s medium range ballistic missiles (MRBMs), there is the potential that these bases may no longer exist to facilitate a retaliatory DCA strike. In this scenario, DCA effectively face the same dilemma as the bomber leg of the nuclear triad.

To be clear, SLCM development should not be designed to create a “usable” first strike capability for the United States. Instead, the development of a small number of low-yield SLCMs should provide a secure, credible second-strike capability against a regional adversary. This will mitigate the destabilizing effects from bomber and DCA forces potentially becoming vulnerable to a theoretical first strike in the future. For these reasons, the United States should consider reintroducing a small number of nuclear-armed SLCMs into the U.S. nuclear arsenal.