From the Document: "Since the early 1960s, the United States has maintained a 'triad' of strategic nuclear delivery vehicles. These include long-range land-based intercontinental ballistic missiles (ICBMs), long-range submarine-launched ballistic missiles (SLBMs), and long-range heavy bombers equipped to carry nuclear-armed cruise missiles and gravity bombs. The number of nuclear warheads carried on these delivery vehicles peaked in the late 1980s, at around 14,000 warheads. It has been declining ever since, both as the United States complies with limits in U.S.-Russian arms control agreements and as it has changed requirements after the Cold War. As of February 2018, the United States had reduced its forces to comply with the New START Treaty, which entered into force in early 2011. [...] Early in the Cold War, the United States developed these three types of nuclear delivery vehicles, in large part because each of the military services wanted to play a role in the U.S. nuclear arsenal. However, during the 1960s and 1970s, analysts developed a more reasoned rationale for the nuclear 'triad.' They argued that these different basing modes had complementary strengths and weaknesses that would enhance deterrence and discourage a Soviet first strike. [...] The United States has reaffirmed the value of the nuclear triad. The Obama Administration noted, in the 2010 Nuclear Posture Review (NPR), that the unique characteristics of each leg of the triad were important to the goal of maintaining strategic stability at reduced numbers of warheads. It pointed out that strategic nuclear submarines (SSBNs) are the most survivable leg of the triad, that single-warhead ICBMs contribute to stability and are not vulnerable to air defenses, and that bombers can be deployed as a signal in crisis, to strengthen deterrence and provide assurances to allies and partners. It also noted that 'retaining sufficient force structure in each leg to allow the ability to hedge effectively by shifting weight from one Triad leg to another if necessary due to unexpected technological problems or operational vulnerabilities.' The Trump Administration also reaffirmed the U.S. commitment to the nuclear triad and to the modernization programs for each of the components of that force structure."
CRS In Focus, IF10519
Congressional Research Service: https://crsreports.congress.gov/