WASHINGTON — What if we fail to prevent an attack?Assume every line of defense against nuclear terrorism is breached: the efforts to lock up nuclear material abroad, to spy out hidden weapons programs, to deter rogue states and capture terrorists, to detect smuggled bombs at the border or downtown — every preventive measure discussed in the previous five installments of this series. Assume someone, somehow, gets all the way through. It only has to happen once. Assume that this someone puts together a crude atomic bomb, of the “Little Boy” type dropped on Hiroshima in 1945, a heavy and awkward device...