I don't know what you guys have heard about the upcoming movie but apparently the three races have become segregated again. Until we learn otherwise, let's just say it was Starlight's fault.

It duzent make sense for it to be after the events of G4; it'd be more appropriate if it was before or during the Celestial Era when Celestia ruled Equestria by herself

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A seemingly perfectly united society that has done away with strife and successfully come together for the common good can fall back into chaos and distrust surprisingly quickly. Britain is one such example, where people seem to be more interested in building walls than bridges, whereas a decade ago it was the other way round.

Oh, and by the way, it's 'doesn't'. D-O-E-S-N-apostrophe-T.
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Clearly something went wrong with the lesson plans!

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Playing devil's advocate here but I doubt Equestria segregating would be Starlight's fault. If anything, she would have most likely fallen back into old habits and turned Equestria into a Marxist cutie-mark free totalitarian police state.

Oh, and used magic to mind-control every single inhabitant in Equestria.

Totally her fault!