When Princess Celestia finds a portal, she trusts her commander to protect it. When there is nopony else to go with her, she brings it upon herself to join her #1 commander. All other soldiers are instructed to go to other cities and towns to help against the war.

When their prisoner escapes and pushes them through the portal, they are in a world much like their own, but different in all the wrong ways.

And for the first time in her career, Rainbow Dash is scared.

(THUMBNAIL NOT MINE. [https://www.deviantart.com/dawnf1re/art/Wardash-580438071)