During the cold war, this was actually a scenario which was not at all unlikely. Both the USSR and the United States were prepared to launch their arsenal as quickly as possible in case the other side showed any signs of aggression. Launching nukes quickly requires loose safeguards. Accidentally triggered world war III was a very common topic. Some cult movies from that era about that topic are War Games, Dr. Strangelove and various James Bond movies. But there are many more examples.

Common triggers are:

• Miscommunication in the chain of command
• Human error while handling the nuclear launch systems
• Crazy supervillain tricking superpowers into nuking each other by staging a nuclear attack

However, if you want your story to take place after the Cold War, then all these scenarios will feel outdated and unlikely. The United States no longer face a threat of mutually assured nuclear destruction (no, North Korea is not a serious nuclear threat to the US mainland), so they no longer need a hare-trigger on their nuclear arsenal.

If you want to revive the cold war era fear of the world constantly being at the brink of nuclear annihilation, you might have to introduce a new nuclear arms race which would again warrant a way to quickly fire (and misfire) nuclear missiles.

During the cold war, this was actually a scenario which was not at all unlikely. Both the USSR and the United States were prepared to launch their arsenal as quickly as possible in case the other side showed any signs of aggression. Launching nukes quickly requires loose safeguards. Accidentally triggered world war III was a very common topic. Some cult movies from that era about that topic are War Games, Dr. Strangelove and various James Bond movies. But there are many more examples.

Common triggers are:

• Miscommunication in the chain of command
• Human error while handling the nuclear launch systems
• Crazy supervillain tricking superpowers into nuking each other by staging a nuclear attack

However, if you want your story to take place after the Cold War, then all these scenarios will feel outdated and unlikely. The United States no longer face a threat of mutually assured nuclear destruction (no, North Korea is not a serious nuclear threat to the US mainland), so they no longer need a hair-trigger on their nuclear arsenal.

If you want to revive the cold war era fear of the world constantly being at the brink of nuclear annihilation, you might have to introduce a new nuclear arms race which would again warrant a way to quickly fire (and misfire) nuclear missiles.