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What is your purpose in wanting to do this? If the story is 99% set in the real world, and then you throw in this one paragraph of alternate history, and then the story goes back to the real world...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/34332 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
What is your purpose in wanting to do this? If the story is 99% set in the real world, and then you throw in this one paragraph of alternate history, and then the story goes back to the real world and the alternate history is never mentioned again ... I don't see the point. The reader will either think you are very confused about history, or they will expect this incident to make a difference. They will expect it to have some relevance or purpose. Readers who are not interested in an alternate history story will be annoyed that you dragged it in, and readers who want to read an alternate history story will be annoyed that you dropped it. There's nothing wrong with mixing genres. You can include a romance subplot in a science fiction story, or have a horror story set in the Old West. But there is something very, very wrong with dragging in another genre for one paragraph and then apparently forgetting that you did it.