Anyone wonder how Rainbow Dash wound up serving Nightmare Moon?

5308099 After Nightmare Moon took over Equestria Rainbow joined due to a combination of feeling like she'd belong with her and on top of that feeling like this could redeem her moment with Shy as she let her down back during the Rainboom.

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Same reason as everyone else: she's the only show in town. Unless Rainbow's got some pressing reason to join whatever resistance there is, she's going to fall into Nightmare's camp by default. Once there, she's going to want to be the best.

Maybe she joined because NM threatened Fluttershy or something.

I think since the rainboom never happened, RD never became the element of loyalty. Thus it was easy for nightmare moon to convince Rainbow to join her. Nightmare moon could have told RD, that she will immediately name her the best flyer/warrior in all of Equestria. In fact since the rainboom never happened, she might not have any loyalties at all and is just staying with Nightmare because she benefits from it.