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Nope. You don't have to give the backstory at all. Sometimes it's just not important to the story. Do make sure the reader understands the setting, but then just tell your tale. That being said...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/41627 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/41627 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Nope. You don't have to give the backstory at all. Sometimes it's just not important to the story. Do make sure the reader understands the setting, but then just tell your tale. That being said, _you_ need to know the backstory. There will be some subtle (or maybe not so subtle) differences in how the characters act, what they do, what the world looks like, etc, all based on what actually happened in the past.