Nuclear war kills. It's local effects are well known. The heat, the shockwave and the radiation with fallout. The far more devastating effect - The Nuclear Winter is not so well known.
Here are some 'Science Fun Facts' of nuclear war and nuclear winter. The reasoning, why during the coldest of the cold war, Presidents Reagan and Gorbachev understood that a nuclear war would most likely wipe the whole humanity from the Earth.
Video: Nuclear Winter | Retro Report | The New York Times