@Sexy Ass  
The enemies are the Imperium of Earth. A powerful earth pony nation,Celestia isn’t mentioned in the description.

Twilight Sparkle… Lunar republic..
 
I don’t even know. Wouldn’t she be fighting for Celestia?

@Pagan  
According to the deviant art page, Rarity’s back at HQ coordinating. I like your idea better, though.

I hope Rarity is inside the tank and refuses to come out because it’s filthy out there and the inside is spotless because she constantly cleans it, obsessively so.
 
Rarity: “AJ no you’ll track dirt everywhere clean yourself off first.”  
AJ: “They are shootin’ at us Rarity let me in!”  
Rarity: “I will as soon as you wipe your hooves.”

Panzer crew ponies AND a sexy Tiger? This is amazing.