“Anything that happens, happens.
Anything that, in happening, causes something else to happen, causes something else to happen.
Anything that, in happening, causes itself to happen again, happens again.
It doesn't necessarily do it in chronological order, though.”
- Douglas Adams, Mostly Harmless

Twilight and Minuette discuss Starlight Glimmer's timeline disruptions.

Twilight just wishes the conversation had happened in chronological order.

When history is rewritten so that Equestria is doomed, it's not a hard choice to try to revert the timeline.

While battling Starlight Glimmer's interference with the timeline, Twilight is faced with a moral dilemma when she lands in a timeline which has weathered Equestria's trials almost as well as her own. How can the lives of those living there be balanced against her vanished world?