You may have noticed that the gender ratio in Ponyville is slightly skewed. Like, five in every six ponies is a mare, skewed. But, it's not just Ponyville, it's all of Equestria. No one knew for sure why, if they ever even thought about that sort of thing; most simply assumed that's just how things turned out.
But, you see, all the stallions are off fighting a war. Not just any war, either.
And as a brand-spanking-new princess and protégé of the magic of friendship, it is now Twilight's responsibility to extend that friendship to a prisoner of war, in the hopes of peace. Celestia knows how this'll go down.