This is wrong on every single level;
The Covenant encountered humans on the colony of Harvest in 2525, and in the next decade proceeded to destroy several hundred worlds that made up the outer colonies including (But not limited) Sargasso, Arcadia, Jericho VII, Chi Ceti IV and more. Reach was the second-last planet to be attacked in mid-2552, the book canon stating this battle to be over in a day and resulting in the destruction of the majority of the UNSC Navy, the SPARTAN-II's and UNSC's primary shipyards. Te game canon instead shows the UNSC to be fighting on the planet for a number of months and engaging in active warfare on a continent the size of Africa with barely a whisper of Orbital Support or reinforcements from Earth; with literally no Naval presence to be sighted despite the appearance of a CSO-Class carrier, a 27 kilometre long vessel. Yet Reach's extensive network of satellites, Naval Defences and outposts do not pick this up or relay it to FLEETCOM, instead letting SPECWARGROUP/3 take care of it. Another discrepancy occurs in that the Pillar of Autumn is not a vessel rated for atmosphere, and was stated to be in combat aboe Reach (with Cortana) at the time of the mission of the same name- raising questions as to how and why Cortana and the Autumn got to the planet's surface for many years until 343i's Data Drops, coupled with the reprint of The Fall of Reach, answered some of the discrepancies. In addition, Halo Wars is set in 2531, making that the first Halo Game, if put in Chronological order.
Going back to your original comment, it would appear that you do not know the story. I would suggest actually paying attention to things before making stupid comments.