We spent the Cold War in perpetual fear that the U.S. and U.S.S.R. would start an intentional nuclear conflict. The truth is, we came far closer to blowing ourselves up with nuclear weapons than we ever came to WWIII.
Nuclear incidents have a bunch of ominous military code names, like Broken Arrow, Faded Giant or NUCFLASH. There are actually dozens of instances like these, but here are five major ones that happened in the U.S. If we were to consider Soviet activity, the list could go on for hours. The Russians either lost a nuclear sub, lost a sub with nuclear weapons on board, had a nuclear sub's reactor melt down, or all three roughly every other week. Kompetentnyh? Nyet.
If that doesn't seem scary enough, there are dozens more incidents like these on the U.S. side alone. We haven't even touched on the Cuban Missile Crisis. The sad lesson is that we have less to fear from naked aggression than we do from incompetence and bad engineering.