Of course, it matters if it is only one bomb, or total war. While the devastation will be localized around the target area, I think the real destruction will be the collapse of civilization. We live in an extremely integrated society. Multiple layers of production, distribution, etc. provide for billions of people. Nuclear bombs will destroy that ability to feed vast numbers of people. Civilization, food production and the transportation system will collapse. What's left may not be able to pick up the pieces for centuries.