Since its inception in 1960, strategic deterrence has been the sole mission of the fleet ballistic missile submarine (SSBN), which provides the nation's most survivable and enduring nuclear strike capability. The 18 submarines of the Ohio class replaced aging fleet ballistic missile submarines built in the 1960s and are far more capable. Carrying 24 Trident missiles each, the Ohios carry 50 percent of all US strategic nuclear warheads and provide the sea-based "leg" of the US strategic deterrent