The Boeing B-52 Stratofortress entered service in 1955, but its Cold War identity was forged in the late 1950s under General Curtis LeMay and his successor, General Thomas S. Power, at Strategic Air Command. Alarmed by Soviet ICBM development and the possibility of a surprise attack that could destroy SAC's bombers on the ground, Power implemented an alert system that changed the calculus of deterrence. On October 1, 1957, SAC began ground alert operations: bombers and tankers at the ends of runways, nuclear weapons loaded, crews living within minutes of their aircraft. By 1960, one-third of the bomber force was on alert. Then came airborne alert: B-52s flying 24-hour sorties, nicknamed HEAD START and later CHROME DOME, with nuclear weapons aboard, ready to divert to Soviet targets on a moment's notice. By 1961, SAC had flown more than 6,000 alert sorties. The ground alert system expanded to 50 percent of the force by July 1961 at President Kennedy's direction. SAC crews joked that they were "waiting for the war to start." The B-52 never delivered the nuclear strike it was designed for. Instead, it became the visible backbone of deterrence—the aircraft that made sure no enemy ever believed they could strike first and survive.