We are not alone!
In exploring and colonizing the galaxy, humanity discovers evidence of eighteen sentient species. Seventeen of them had not developed interstellar travel. Those were destroyed by the species that did reach the stars. That space-faring eighteenth decimated the human colony on the Semi-Autonomous World Troy.
A Marine Force Recon platoon sent to investigate is wiped out almost to the last man. In reaction, the North American Union assembles the largest army seen since the major wars of the 20th Century.
A Marine Corps Combat Force is sent to "Kick in the door," backed up by a four-divison Army corps to take the planet back. The initial landing is unopposed. It isn't until the fleet carrying the Army corps is approaching Troy that the enemy strikes, with devastating effect.