It is the 26th century, and humanity is dying.
Twenty years ago, the backwater colony world of harvest went silent. At first, a single scout ship was dispatched to check up on the colony. Attacks from terrorist groups were not uncommon, and an FTL communications system was a juicy target for such groups. The scout ship arrived, sent a single message to confirm that it was in the system, and then promptly disappeared. Following that, a nearby battlegroup of three warships was dispatched to the planet. A single destroyer and a pair of escorting frigates, a force that was more than enough to find out just what had happened to the planet. Almost two months passed before the fleet returned, when the sole surviving destroyer limped back to the stronghold world of Reach and reported what it had found.
The fleet had dropped out of FTL travel right above the planet. Or what remained of it. The world had been turned from a paradise of green grasslands and blue oceans, to an inhospitable ball of glass and ice. A single ship of alien origin sat in orbit and send only a single message to the human vessels.
"Your destruction is the will of the gods, and we are their instrument."
The resulting battle lasted all of 30 seconds and resulted in the unprepared fleet being decimated. The sole surviving vessel, the destroyer Hercules, only survived the engagement as it's slipspace drive was still spooled up and ready to go. And even then, the battle had crippled it, forcing it to use its backup reactor in order to return to Reach.
This first engagement would set the general trend for naval battles throughout the war. Trial and error would lead to tactics that would give humanity better odds in battle, and the genius of officers like the great Admiral Cole could even achieve a positive kill/loss ratio in certain battles. Massive shipyards mass-produced warships around the clock, and all of humanity's military might was consolidated under the command of the UNSC.