Equestria is fighting her first World War after the 2nd Industrial revolution. She hasn't fought a war in a while. Before his death, Discord enlists the help of several creatures that are all too familiar with the concept.
When a mysterious figure steals the Element of Magic from the Tree of Harmony, Twilight chases him into a strange realm she does not understand. Its name: Eorzea.
The wars humanity remembers may not be the most memorable after all. The past is full of mysteries we thought were solved, and men we thought were dead live yet.
The Mane Six are transported to Earth by one of Twilight's experimental teleportation spells. Unfortunately for them, they just happen to appear on a hill overlooking the Battle of Agincourt.