The primary advantage of the SLBM over other delivery nuclear systems was derived from the comparative invulnerability of its submarine launch platform. In theory, a nuclear-powered ballistic missile submarine (SSBN) was secured by its nearly silent operation and an endurance limited only by its crew's capacity to stay at sea.
The development of the SLBM overcame daunting technological challenges including concerns over proper guidance, warhead size, and underwater launches. Unlike land-based missiles, the dynamic launching point of the SLBM demanded the development of sophisticated inertial guidance systems that could mark the weapon's precise launch point. Furthermore, most SLBMs used stable, solid fuels, both for safety at sea and to guarantee the ability to launch without vulnerable missile-fueling time. Additionally, limited space and weight required the development of comparatively efficient warheads. Finally, underwater launching avoided the time and vulnerability entailed in surfacing.
The world's first fully effective SLBM to overcome all these obstacles, the U.S. Polaris A-1 missile, grew out of the U.S. Navy's early postwar ambivalence about nuclear weapons. The navy's initial position, which found full expression during the 1949 Revolt of the Admirals (when the U.S. Air Force had sole delivery capability) was that atomic weapons were ruthless and barbaric. This position abruptly changed in the early 1950s as atomic weapons were first deployed on carrier-borne aircraft and the navy's Regulus cruise missile. The February 1955 Killian Report's recommendation to push forward with both sea- and land-based intermediate-range ballistic missiles (IRBMs) gave the necessary impetus to the development of an SLBM. After an initial flirtation with a solid-fueled version of the U.S. Army's Jupiter missile, the navy's Special Projects Office pursued the Polaris program beginning in late 1956. After surmounting numerous technological obstacles and sixty-two test launches, including the first underwater launch on 20 July 1960, the Polaris became operational at the end of that year with sixteen missiles aboard the SSBN George Washington. The Polaris A-1 was capable of carrying a 600-kiloton (kt) warhead with a firing range of 1,200 miles.
The Polaris A-1 was supplemented in 1962 by the Polaris A-2, which had a slightly longer range, allowing for Mediterranean operation, and a larger warhead. The Polaris A-3, which became operational in 1964, was the first multiple reentry vehicle (MRV) system. The A3 had three 200 kt warheads, providing strike capability against projected Soviet antiballistic missile (ABM) technology, and a range of 2,500 nautical miles. The Poseidon C-3 SLBM, deployed in 1971, used ten 40 kt warheads in a multiple independently targeted reentry vehicle (MIRV) designed to overwhelm Soviet ABMs around Moscow and came equipped with increased accuracy providing very limited capability against hardened targets. The final iterations of U.S. SLBMs, the Trident C-4 and Trident II D-5, deployed in 1979 and 1989, respectively, provided the hard-target kill capability that had been long sought-after.
Soviet SLBM development followed a similar track but lingered several years behind equivalent U.S. technology. The first Soviet SLBM, the liquid-fueled SS-1b, a modification of the land-based SS-1 Scud, could only be fired from a surfaced submarine and had a range of only 90 miles. Follow-on systems in the 1960s, including the SS-N-6 IRBM, could be fired while submerged and carried megaton-range warheads but were liquid-fueled and grossly imprecise, with accuracies measured in kilometers. The next decade saw intercontinental range (SS-N-8) and MIRV development (SS-N-18). Only in the 1980s did the Soviets deploy technologically equivalent solid-fueled missiles, the SS-N-17 and SS-N-20.
While France developed its own domestic solid-fueled SLBMs from the 1970s on, Britain adopted the U.S. Polaris in the late 1960s. After indigenously adapting the Polaris warhead in the Chevaline Program to defeat Soviet ABMs, Britain again turned to the United States in the 1980s with the purchase of the Trident SLBM. The only other SLBM-armed nation, the People's Republic of China (PRC), deployed a solid-fueled, single-warhead weapon in the late 1980s.
The principal effect of SLBMs on nuclear strategy was to provide the technological means for a credible last-ditch deterrent. In the United States, the low accuracy and small warheads of the earliest missiles pushed the navy in 1957 to advocate finite deterrence, which called for maintaining only a small and secure second-strike capability, a role for which the Polaris was perfectly suited. Although this idea did not become the basis of deterrent policy, SLBMs formed the most secure (if least accurate and slowest reacting) leg of the American and Soviet nuclear triads. Their relative economy, compared to manned bombers and ICBMs, provided France and Britain with a sufficient nuclear deterrent, in the latter's case as the only operational strategic system after 1969. However, in reality these advantages were offset by relatively low accuracy and yield. Furthermore, SSBN maintenance requirements would have found a substantial number of them in port, vulnerable to surprise attack.
International Institute for Strategic Studies. The Military Balance: 1990–1991. London: Brassey's, 1990.; Martel, William C., and Paul L. Savage. Strategic Nuclear War: What the Superpowers Target and Why. Westport, CT: Greenwood, 1986.; Spinardi, Graham. From Polaris to Trident: The Development of U.S. Fleet Ballistic Missiles. Cambridge: Cambridge University Press, 1994.; Zaloga, Steve. The Kremlin's Nuclear Sword: The Rise and Fall of Russia's Strategic Nuclear Forces, 1945–2000. Washington, DC: Smithsonian Institution Press, 2002.