IN THE PENTAGON these days, those who plan for Doomsday have a new nightmare: no longer yesteryear’s dread of one big nuclear foe, but of several at the same time. What if, asks one official, Russia attacked a NATO country, drawing America in to defend Europe; then China seized on America’s distraction to invade Taiwan; and then North Korea decided to attack the south? Three wars; three sets of friends and allies; three unpredictable nuclear crises. Could America handle them all?