From Halopedia, the Halo wiki
2525 was a year in the 2520s in the 26th century. It was a year that saw the start of the Human-Covenant War, which would become a twenty-seven year long engagement between the Covenant Empire and the United Nations Space Command. The war began after first contact between the belligerents at the human Outer Colony of Harvest. The emergence of the Covenant threat prompted the training of the Spartan-II supersoldiers to be accelerated and a group of them were selected for their very first live combat mission in September of this year, an operation to capture the United Rebel Front colonel, Robert Watts.