ACCORDING TO Defense Secretary Robert Gates, every senior government leader is kept awake at night by "the thought of a terrorist ending up with a weapon of mass destruction, especially nuclear." This is, I suppose, understandable. It was in 1995 that the thoughtful analyst Graham Allison declared that "in the absence of a determined program of action, we have every reason to anticipate acts of nuclear terrorism against American targets before this decade is out." Unabashed, he maintained in an influential 2004 book that "on the current path, a nuclear terrorist attack on America in the decade ahead is more likely than not." And it was on "60 Minutes," on Nov. 14, 2004, that former CIA analyst Michael Scheuer assured his rapt interviewer that the explosion of a nuclear or dirty bomb in the United States was "probably a near thing."