The nuclear bomb dropped on the Nagasaki of Japan (few decades back) is the example of nuclear destruction the human has seen then. The new borns till now in those areas are having problems in their body parts now also, after few decades.

Some threats for humanity come from humanity itself. The scenario that has been explored most is a nuclear war or another weapon with similar possibilities. It is difficult to predict whether it would exterminate humanity, but very certainly could alter civilization, in particular if there was a nuclear winter.