Following Princess Twilight's ascension to the throne, the military's standards are upped all over Equestria. The Wonderbolts are part of the military, despite the fact that their involvement in military operations has waned over the centuries. Getting out combat uniforms is something new for Rainbow Dash.

Set after the events of The Ending of the End.
Edit: I just realized that I initially said this was set after the events of The Last Problem... Sorry if that made any confusion