sentences:  They agreed to use the Moscow–Washington hotline, which was upgraded at the same time, for any communications.The decision not to restart on-alert bomber missions was also a reflection of the strategic decline of manned nuclear weapon delivery in favor of unmanned delivery via ICBMs, which had already eclipsed the number of bombers in the United States by April 1964.Following the Palomares and Thule accidents—the only cases where the conventional explosives of U.S. nuclear bombs accidentally detonated and dispersed nuclear materials—investigators concluded the high explosive (HE) used in nuclear weapons was not chemically stable enough to withstand the forces involved in an aircraft accident.