The Battle of Primus was a military engagement between the United Galactic Alliance and the Thanix Assimilation during the early years of the Genesis War. The engagement began shortly after the Battle of Domusan, the failed attempt by UGA forces to cripple Thanix forces on the colony. The UGA retreated from Domusan to Primus, a barren Forerunner world a few light years away where UGA forces had built a base a few days earlier. There, the remnants of the force that attacked Domusan would attempt to repair and re-arm for another strike, however, this would be impossible. Unfortunately for them, Assimilation forces tracked them to the planet and began burning the planet.