The link below contains an exceptionally interesting article by Bridge Colby. It argues for "expanded deterrence" in the face of a terrorist-inspired nuclear attack on the US or its allies. Readers are likely to find much that is uncomfortable in Bridge's analysis, especially its dramatic expansion of the circle of those whom the US might hold responsible for such an event. Nonetheless, the article constitutes a serious attempt to analyze the implications of this question and to develop serious policy options.
One question I would have is whether, in embracing the expanded deterrence thesis, US policy makers would also concede similar right of action to other countries, for example Russia and China, who might also come under terrorist attack. Some also argue that the dangers are not immediate. Access to fissile material and the problems of miniaturization of nuclear devices represent not inconsiderable challenges for non-state actors like Al-Qaeda.