David Seed’s magisterial analysis of nuclear terrorism in fiction has confirmed one large suspicion I had about such books: The warhead MacGuffin is, far more often than not, stolen or donated by a sinister benefactor instead of being scratch-built. The Sum of All Fears is one big exception, just as how Red Storm Rising has Iceland invaded and the war staying completely conventional from start to finish. Because I love overthinking stuff like this (in violation of the wise words of literary theorist Mr. Hippo that not every story has to have significance), I have a few possible theories.
Theory 1 states that this is an example of being, even accidentally, technically reasonable. There are large practical issues with constructing even the simplest Little Boy-esque designs. The biggest and most obvious is appropriate fissile material, but there’s more that’d be hard to do and harder to do in secret. Of course, most authors would probably be going with their gut telling them it’s just easier to skip that step. Seeing so many writers get absolute basic technical details wrong makes me think it’s more a broken clock being right at that moment than anything else.
Theory 2 is less generous and states that it’s because it’s easy to write, means you don’t have to research, and can just say “here, they got a nuke.” Even The Sum of All Fears, as (over)-researched at it was, did this in a way by almost literally dropping the Israeli bomb into the laps of the antagonists.
Theory 3 is simply the result of bandwagoning. Because everyone else is having the nukes being sold or donated, the authors are simply writing what they know. Tom Clancy is weird in that while a lot of people adopted his themes, his exact style is not replicated nearly as much (and understandably so)
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The warhead issue symbolizes something I’ve noticed about fiction: Realism is often not really that much of an asset. Roughly speaking, a lot of the people won’t know any better, many of the ones who do know won’t care, and a giant subset of those who do care will find issue no matter what. This combined with thrillers almost always succeeding and failing based on execution and not concept means my advice would just be to write what you’re comfortable with.
My personal take is this: Since I’ve been reading so much on the topic, and since I find the stolen/gifted nuke overused, I feel like I’d have the warhead be scratch-built if I wrote a book on the subject.