In the film: After trying to negate a nuclear war started by a rogue general, the government and President are unable to prevent one of the deployed bombers from being found or recalled, so he destroys a missile silo, triggers a Soviet doomsday machine and most likely annihilates all of human life.

In the book: They still can’t reach the last bomber, but due to the bomb getting damaged as dropped, it fails to hit the target, and doesn’t detonate to its full capacity.

After that, the world starts to put arrangements in place to prevent this from ever happening again.