Based on Equestria at War mod.
After the Equestrian frontline had collapsed against the Changeling army in the Coltfoalnian Campaign and the Breakthrough at Mariposa. The remnant of the Equestrian Royal Armed Forces(ERAF) abandoned Northwestern Equestria to defend the vital region of Central Equestria.

An earth pony was drafted into ERAF and was assigned to the newly formed 3rd Canterlot Garrison with the obvious task to defend the Jewel of Equestria, Canterlot.

Unbeknownst to him, he will witness and experience the largest battle in the war.