While current United Kingdom's nuclear forces—four Trident s—are strictly under UK national control, they had two distinct roles under the SIOP. The first was part of a UK-only retaliatory response to a nuclear attack, whether a full strategic strike, or a limited tactical strike. The second role was one in which the Royal Navy participated in the SIOP, in effect becoming an extension of the U.S. Navy's Trident submarines. This role was to be part of a NATO response to a Soviet nuclear strike. The Royal Navy's contribution to the SIOP was small. The four "Vanguard" submarines could strike a maximum of 512 separate targets; equivalent to 7% of the total U.S. nuclear strike capacity.