The threat of nuclear weapons has been lingering for a while now. There are multiple countries that have the technology now. There may come a day when one of these countries decides to use a nuclear weapon. This day would most likely mean the end of humanity as we know it. It could very well end all human existence with the fallout.
Nuclear weapons are almost the sole reason for the lack of large scale war since the end of WWII, and have done a great service to peace on earth. While humanity will always fight, and there's no end to the various regional conflicts in the world, the scale of war and its affect on civilization pails in comparison to the effects of the total wars fought throughout history.