Eventually the rescue of downed American pilots became the second most important submarine mission after the destruction of Japanese shipping. Initially, the rescue submarines met several obstacles, most important of which was the lack of communication between the submarines and aircraft in the area; this led to several Lifeguard League submarines being bombed or strafed, possibly including the sinking of and by American planes. As fighting in the Pacific theater intensified and broadened in geographic scope, the eventual creation of Standing Operating Procedure (SOP TWO) led to several improvements such as the assignment of nearby submarines before air attacks, and the institution of reference points to allow pilots to report their location in the clear. After the capture of the Marianas, targets such as Tokyo, about 1,500 mi (2,400 km) north of the Marianas, were brought within range of B-29 attacks and Lifeguard League submarines began rescue operations along their flight paths. Submarine lifeguards spent a combined 3,272 days on rescue duty and rescued 502 men. Famous examples include the rescue of 22 airmen by the , and the rescue of future U.S. President George H. W. Bush by the . Cold War (1945–1991) After WWII, things continued along much the same path until the early 1950s. Then, a revolution that was to forever change the nature of the submarine arm occurred. That revolution was . Towards the "Nuclear Navy" The was the first nuclear-powered submarine. Nautilus put to sea for the first time on 17 January 1955, transmitting the historic message, "Under way on nuclear power." Up until that point, submarines had been torpedo boats tied to the surface by the need to charge their batteries using diesel engines relatively often. The nuclear power plant of the Nautilus meant that the boat could stay underwater for literally months at a time, the only operational limit being the amount of food that the boat could carry. With resupply by mini-subs, even this could be overcome. The final limits would be for replacing equipment that wears out, the fatigue limit of the hull, and crew morale. Strategic deterrence Another revolution in submarine warfare came with . Nuclear-powered like , the George Washington added strategic ballistic missiles making the nuclear triad. Earlier Regulus missile submarines were diesel powered. Their cruise missiles required the boat to surface in order to fire, and were vulnerable to air defenses in an era when there were