Trident is the current nuclear deterrence programme of the United Kingdom. It has been in service since December 1994 and consists of four Vanguard-class submarines armed with Trident ballistic missiles that are able to deliver multiple independently targetable thermonuclear warheads.
Based 25 miles from Glasgow at the Clyde Naval Base on the west coast of Scotland, Trident is operated by the Royal Navy and has been the only nuclear deterrent system of the United Kingdom since free-fall bombs were decommissioned in 1998.
At least one submarine is always on patrol, providing the United Kingdom with a “continuous at-sea deterrent”. Each one is armed with up to 8 missiles and 40 warheads, though their capacity is much greater.
According to the Ministry of Defence, the purpose of Trident is to “deter the most extreme threats to our national security and way of life, which cannot be done by other means”.