In 1982, the Polaris Sales Agreement was amended to allow the UK to purchase Trident II missiles. Since 1998, when the UK decommissioned its tactical WE.177 bombs, the Trident has been the only operational nuclear weapons system in British service. The delivery system consists of four s based at HMNB Clyde in Scotland. Each submarine is armed with up to sixteen Trident II missiles, each carrying warheads in up to eight multiple independently targetable reentry vehicles (MIRVs). With at least one submarine always on patrol, the "Vanguards" perform a strategic deterrence role and also have a sub-strategic capability.