At the end of the Second World War the United States, Britain and the then USSR were allies. However that belied an uneasy tension between the conflicting views of the democracy of both the USA and the UK, and the Communist ideology of the USSR. By the late forties this uneasy tension had deepened, and the Cold War had begun.
Of the western partnership the USA was the stronger and richer of the two allies. America had nuclear weapons. Britain did not, but was unwilling to accept its diminished role in the world. By the 1950s the Conservative Government led by Prime Minister Harold Macmillan was determined to remain inside the elite club of world powers. Britain began to develop the hydrogen bomb, known as the H bomb, and the bombers to carry such a weapon.
In 1957 the Russians launched Sputnik into space. The West was stunned. The Russians now had the capacity to launch a weapon from space. The warning of a strike would be four minutes. American cities and military installations were now targets within the range of Russia’s weapons. Britain’s planned bombers were obsolete.
The American solution was three stranded, one of which was to put warheads into submarines from which they would be launched. The USSR did not have the capability of detecting submerged submarines.
From the American point of view, if any of their land locations was under threat of an attack, the potential reprisals unleashed from beneath the waves would act as a deterrent.
This method of delivering warheads was called Polaris.