Nightmare Moon just wanted to rule in peace. Sure, the changelings weren't exactly the friendliest neighbors Equestria had, and the people of the east, from the griffons to the riverponies, were future threats to her reign that had to be dealt with, but she was confident that all would go well in the end. Unfortunately, a Daybreaker from an alternate universe was absolutely not a variable Nightmare Moon had accounted for.
(Mostly based on the world of Equestria at War, a My Little Pony: Friendship is Magic mod for the WW2 grand strategy game Hearts of Iron 4 by Paradox Interactive, and also a few headcanons that I thought would be fun.)