The year for this game will be 2612. There are no wars going on and for the first time in a while there has been peace among the colonies. However a discovery was made and that discovery was kept secret from everyone outside of ONI. That discovery was a device that was capable of transporting matter from one planet to another, instantaneously. This device was called a Stargate, as that is what it was called on the Forerunner base it was found in. The artifact was shipped out and kept under heavy guard by ONI has they put teams together to explore the unexplored worlds that are on accessible through the Stargate.
Since its discovery, no other stargates were found. It is unknown where other gates are but we were able to pull a few addresses out of a Forerunner database and it appeared that it was made by the Precursors but that was never confirmed. The teams are coming together and although it is on a secret ONI base that doesn't mean that only ONI officials are assigned to a team to explore what else is out there. From Spartans, to ODST, to Marines, Army Soldiers, Sailors and beyond anyone can take part, so long as you were chosen by ONI to do it.