A species similar to the Krogans has lived on the planet Geilgar IV in the Geilgar System for millions of years. With a life span of 150 and a mind much smarter then a human. These creatures have managed to capture an entire system but have yet to be able to leave their galaxy. One day an Citadel Research team finds the area. In a few seconds they are destroyed by the Geilgarians. For the Geilgarians, the First Contact War just started. Unknown to the Citadel the Geilgarians have an army of two trillion.

A whole star system has nowhere near enough resources for an army of two trillion.

Given that they have only expanded into one system, their space capacity is not enough to defeat a Citadel flotilla and will be easy prey. Once orbital superiority is achieved, Citadel does what the Covenant did to the UNSC. The Geilgarians might win on the ground but that's useless if you get glassed from orbit.

Unless you mean a whole celestial sector, like a nebula or other feature. Then we're getting somewhere.

Also, what do you mean by "haven't left their own galaxy"?