The ballistic missile submarine HMS Vanguard, lead ship of her class, arrives at HMNB Clyde in Faslane, Scotland, following a patrol in 2010. Ballistic missile submarines like Vanguard were introduced during the Cold War, and serve as the third leg of the nuclear triad in countries which also operate nuclear-armed land-based missiles and aircraft. Their intended role in the nuclear triad is to provide a second-strike capability in a defensive capacity, or to offer nuclear-armed nations a first-strike weapons platform. In addition to the first- and second-strike capabilities, Vanguard carries a handwritten letter from the Prime Minister of the United Kingdom which contains orders on what action to take in the event that an enemy nuclear strike has destroyed the British government.Photograph: Tam McDonald