It's the end of the Solar War, Daybreaker has been killed, which also means Princess Celestia is dead.

In this post-Daybreaker Equestria, the crimes against creatures that took place must be dealt with. Ponies will pay for the atrocities they did to the thestrals.

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This story takes place in the Equestria at War universe for Hearts of Iron 4. Here's the groups FimFiction page.