Second strike capability is essential to nuclear deterrence. Second strike provides the capacity to retaliate after a first strike, creating stability in the international system. Bernard Brodie defined stability as “achieved when each nation believes that the strategic advantage of striking first is overshadowed by the tremendous costs of doing so.” Stability is achieved when an opponent’s strategic calculus is altered and it sees no advantage to striking first. In Thomas Schelling’s words, “There is a difference between a balance of terror in which either side has the capacity to obliterate the other, and one in which both sides have the capacity no matter who strikes first.” In the nuclear triad, the ballistic missile submarine (SSBN in U.S. parlance) is the primary guarantor of second strike capabilities. Because SSBNs are so difficult to locate, they would likely survive a first strike on a nation’s military assets and could launch a retaliatory strike. SSBNs, in short, have heretofore guaranteed “the stability of the balance” in the international system. Three major trends, however, may affect the survivability of SSBNs, the security they provide their countries, and, therefore, what stability they ensure among competing states. SSBNs are proliferating, they are getting quieter, and methods for detecting SSBNs are improving. These three trends set the nuclear powers up for a competition that may diminish the stabilizing role SSBNs have historically played. Proliferation The United States and the Soviet Union were the first states to launch a submarine-based nuclear strike capability, but France, the United Kingdom, and China all developed different types of SSBN second strike as well from the 1960s to the 1980s. Recently, India, Pakistan, and North Korea have begun developing an undersea-based deterrent. Some suggest that Israel has built second strike modifications to its ThyssenKrupp-produced submarines. Each country maintains its capabilities relative to a rival: the United States, UK, and France to Russia; China and North Korea to the United States; Israel to its regional rivals; and India to China and Pakistan. By the 2020s, when multiple new generations of SSBNs come online, the world will face a greater distribution of second strike capabilities than ever before, in an increasingly multipolar system, designed for a multiplicity of rivalries. Noise Historically, a submarine has lived and died by its noise profile. Engine noise, crew noise, and other equipment noise can alert sub-hunters to the presence of an SSBN, or create a signature that puts the platform at risk. All the SSBN states are working to make their submarines quieter, with technologies ranging from engine quieting and hull coatings to the application of air independent propulsion, which makes diesel submarines much quieter and faster, to SSBN-sized vessels. Though nuclear-powered SSBNs are the gold standard due to their longer potential deployment times, these quieting technologies will increase the lethality of diesel submarines. Reducing the noise of SSBNs creates a short- to medium-term challenge to submarine detection— acutely affecting states with weaker sub-hunting abilities. Detection In the long term, robots, artificial intelligence, computing power, big data, and the use of light and novel spectra will further enable submarine detection. Once operational, DARPA’s Sea Hunter sub-hunting unmanned vessel will track diesel subs independently, and its Distributed Agile Submarine Hunting will turn the sea into a network of detection devices. Meanwhile, experiments in big data are making fuzzier detection methods more accurate, and sharpening passive detection. Over the long term, the undersea arms race may transform the value of SSBNs. If SSBNs become more detectable, states will have less certainty in their survivability, and the stability of the balance may diminish. Stability of the Balance As states jockey for an edge in undersea warfare, they will likely undermine—at least temporarily—the security of rivals’ SSBN fleets. Advances in technology will favor those with the highest end computing and robotics abilities, but will eventually proliferate among the major powers. This dynamic has the potential to create oscillating periods of stability in the balance—a system defined by periods of uncertainty as to whether “both sides have the capacity [to obliterate each other] no matter who strikes first.” If the benefits of stealth continue to decrease, states will need to start thinking about how to guarantee stability in a system that has so far relied significantly on stealth. As the United States looks forward to an era where its network of unmanned undersea sensors, vehicles, and sonobuoys provides greater visibility than ever before, rivals may feel insecure with their deterrent—and they may grow more aggressive to ensure security because of it. Will undersea warfare look more like surface warfare and aerial combat? Will U.S. rivals move more aggressively toward first use policies? U.S. strategists should imagine what the world after secure second strike looks like, or the United States may find itself in the paradoxical position of losing its security by winning the arms race.