Serving as a US Air Force launch control officer for intercontinental missiles in the early Seventies, First Lieutenant Bruce Blair figured out how to start a nuclear war and kill a few hundred million people. His unit, stationed in the vast missile fields at Malmstrom Air Force Base, in Montana, oversaw one of four squadrons of Minuteman II ­ICBMs, each missile topped by a W56 thermonuclear warhead with an explosive force of 1.2 megatons—eighty times that of the bomb that destroyed Hiroshima. In theory, the missiles could be fired only by order of the president of the United States,…