A submarine-launched ballistic missile (SLBM) is a ballistic missile capable of delivering a nuclear warhead that can be launched from submarines. Modern variants usually deliver multiple independently targetable reentry vehicles (MIRVs) each of which carries a warhead and allows a single launched missile to strike several targets. Submarine-launched ballistic missiles should not be confused with submarine-launched cruise missiles, which operate in a different way.
The first practical design of a submarine-based launch platform was developed by the Germans near the end of World War II involving a launch tube which contained a ballistic missile, which was towed behind a submarine. It was never even tested due to the end of the war, but the scientists that worked on it went on to work for the USA and USSR on their SLBM programs. These and other early SLBM systems required vessels to be surfaced when they fired missiles, launch systems eventually were adapted to allow underwater launching in the 1950-1960s, with the United States making the first underwater launch of a Polaris on 20 July 1960. The Soviet Union made its first successful underwater launch of a submarine ballistic missile on 10 September 1960.
Ballistic missile submarines have been of great strategic importance for the USA and Russia and other nuclear powers since the start of the Cold War, as they can hide from reconnaissance satellites and fire their nuclear weapons with virtual impunity. This makes them immune to a first strike directed against nuclear forces, allowing each side to maintain the capability to launch a devastating retaliatory strike, even if all land-based missiles have been destroyed. This relieves each side of the necessity to adopt a launch on warning posture, with its grave attendant risk of accidental nuclear war. Additionally, the deployment of highly accurate missiles on ultra-quiet submarines allows an attacker to sneak up close to the enemy coast and launch a missile on a depressed trajectory - a very close range attack which will hit its target in a matter of minutes, thus opening the possibility of a decapitation strike.
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