Based on Visiden Visidane's Upheaval series, which begins in Breaking Point, this story is non-canon with that story but written as a fan tribute to the series. I highly recommend checking out the canon series if you're a fan of adventure and military exploits.
Equestria is a peaceful realm, but one that has been kept in the dark about the price of that peace. The Heartland, as it is called, has been protected for generations by the Barrier Lands, a realm outside of Equestria that is occupied by ponies who make up a large army known as the Legion, controlled by the brother of the Princesses, Prince Terrato. This Legion has been fighting an ongoing war for centuries against a host of deadly enemy species that would gladly destroy Equestria if not for the Legion's efforts to keep them at bay.
After a series of events leads Twilight Sparkle and her friends to discover all this, the Barrier Lands and the Heartland are reunited, and the peaceful Equestrians told the truth. However this reunification is not entirely smooth and has with it a terrible price of its own; the draft. Equestrains who have never known a day of violence in their lives must now serve in the Legion, whether they want to or not.
Trixie Lulamoon, Blossomforth, and Coco Pommel are three such ponies, drafted into the Legion and sent to the Western Barrier Lands, where the dangerous bear-like race known as the ursans rule a vast realm of mountains and forest. Though their posting at Beartrap Fortress is meant to be a quiet one, out of the way of the main fighting, these three mares suddenly find themselves stuck in the forefront of an unexpected invasion. Ursans gather in numbers not seen in centuries under the rule of a new warchief, and it seems Beartrap Fortress is directly in the path of this new assault.
To make matters worse it seems there is an even deadlier new threat lurking in the shadows behind this invasion, one that could change the very dynamic of the Legion's struggle to protect Equestria.
Can a single company of raw, inexperienced Equestrian draftees make a difference in such a battle? Its time for the Legionnaires of Equestria to find out what they're made of, one way or another.