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.