During the Cold War the United States developed the Trident class ballistic missile submarine (SBN) to replace the aging fleet of forty-one Poseidon ballistic missile submarines. Each of the eighteen Trident class submarines built to carry the mantle of strategic nuclear deterrence was extremely large and quiet with tubes for twenty-four ballistic missiles. Following the breakup of the Soviet Union and the end of the Cold War, the United States conducted a review of its nuclear posture, which determined that only fourteen of these submarines were necessary to meet the needs of U.S. national security.