The Everything Draft
I often find it useful to think of what I’m working on in first draft as an “everything” draft. I’m throwing everything in including the kitchen sink. If I’m doing research, I throw it all in. If I have backstory, throw it in. If I have a character sketch, put it in. In a way, this first draft is all for me. It’s not meant for a reader to see. It isn’t fun to read. It’s boring and it has way TOO much information, but it helps me deal with the impulse to make sure I have everything I need.
Then when it’s time to sit down for a cleaner draft, I can cut freely because I have the everything draft. If I ever cut too much, I can go back to my everything draft and retrieve it. (Mostly, there’s not such thing as cutting too much, BTW.) A reader doesn’t want to know everything about the Russian Revolution, for instance. But you might need to know in order to write a good book set in that time period.
An everything draft is like outtakes from a movie. You don’t show them for the regular movie screenings. The regular audience doesn’t hear the director’s comments. Those are only for the super fans. And sure, you can add some extra historical details in an author’s note in the back if you want. (My experience is that readers never read them, but maybe librarians?)
I have a general rule of thumb for sff that you can only do one paragraph of backstory/world-building before you go back to the plot that is moving things forward from this moment. This is so that readers don’t get bored and feel like they’re never going to find out what happens next. For adult history, you might be able to get away with more than one paragraph, but if it’s more than a full page, I recommend you reconsider for a general audience.
If in doubt, leave it in the everything draft.