14 ballistic missile submarines (SSBNs), 4 guided missile submarines (SSGNs) (3 in commission) – fast attack submarines (19 in commission, 1 delivered, 1 fitting out, 7 under construction, 2 on order) – fast attack submarines Fast attack submarines The U.S. currently operates three classes of fast attack submarine: the , , and es. There are 34 Los Angeles-class submarines on active duty and 28 retired, making it the most numerous nuclear-powered submarine class in the world. Except for , submarines of this class are named after U.S. cities, breaking a Navy tradition of naming attack submarines after sea creatures. returned to the use of sea creatures as names, but the second and final ships of the Seawolf-class, and , were named for a U.S. state and a U.S. president, respectively. Ships from the Virginia-class except for are named after U.S. states, a convention traditionally reserved for battleships and nuclear missile submarines. The final 23 boats in the Los Angeles class, referred to as "688i" boats, are more quiet than their predecessors and incorporate a more advanced combat system. These 688i boats are also designed for under-ice operations: their diving planes are on the bow rather than on the sail, and they have reinforced sails. Ballistic and guided missile submarines The U.S. has 18 Ohio-class submarines, of which 14 are Trident II SSBNs (Ship, Submersible, Ballistic, Nuclear), each capable of carrying 24 SLBMs. The first four which were all equipped with the older Trident I missiles have been converted to SSGN's each capable of carrying 154 Tomahawk guided missiles and have been further equipped to support Special Operations (SEALS). If the maximum of 154 Tomahawk missiles were loaded, one Ohio-class SSGN would carry an entire Battle Group's equivalent of cruise missiles. Ballistic missile submarines (SSBNs or boomers in American slang) carry submarine-launched ballistic missiles (SLBMs) with nuclear warheads for attacking strategic targets such as cities or missile silos anywhere in the world. They are currently universally nuclear-powered to provide the greatest stealth and endurance. They played an important part in Cold War mutual deterrence, as both the United States and the Soviet Union had the credible ability to conduct a retaliatory strike against the other nation in the event of a first strike. This comprised an important part of the strategy of Mutual Assured Destruction. In order to comply with arms reduction against the START II treaty, the U.S. Navy modified