Author's Note: This takes place (mostly) on Earth one year after the events of Halo 2, but starts off eight years before, for a bit of background. After a few chapters showing important scenes, it will jump to the year 2553 and continue from there. If anything in here clashes with any event(s) from Halo 3, assume this is a reality in which Halo 3 has not yet been announced as existing. If nothing clashes...assume it's going on behind-the-scenes.
The year is 2553; one year after the Prophet of Regret first arrived on Earth, possibly looking for Forerunner installations. Some minor exploration revealed to the Covenant that this was, in fact, the home-world to humankind. When the Earth's military attacked, the Prophet's small fleet was forced to retreat to Delta Halo, destroying the large African city of New Mombassa in the Slip-Space rift left in its ship's departure.
In the year since the Covenant's discovery of Earth, their home, High Charity, has been completely dominated by the Flood. Contact with the Master Chief (stowed away on the Ark) has all but stopped, and the Covenant have gained a significant foothold on the planet. Almost daily, they send smaller ships from deep in space to land near important landmarks and do as much damage as humanly possible.
The MAC Guns orbiting the Earth have kept the Covenant's larger warships at bay, but are unable to target and destroy the smaller landing-parties. Instead, the Marines are on call every hour of every day, waiting for the next strike. Even while the remaining Elites and a handful of Grunts and Hunters have taken a stand against the Prophets with humankind, their numbers still do not match that of the Covenant armada.
It's been made abundantly clear: the Covenant's full-scale, all-out invasion of Earth has begun, and even in their state of Civil War, Earth's defenses are still incapable of holding them for long. But some hope has survived. A Spartan program has been going on in secret on Earth for almost two decades, and the time for training is almost passed. At the end of a long line of failed experiments lies the small but formidable Zulu Company, the only testament to the true worth of new, but risky, geneological and cybernetic testing. The Master Chief has served as a one-man-army for years, but he is still just that: one man. His efforts in space to bring the war to the Covenant have been very successful, but they have not helped the war at home. Abducted seven years before Regret came to Earth, these new Spartans - these last Spartans - may be humanity's last stand before extinction.
Author's Note (last seemingly-pointless one for a while): I enjoy constructive criticism and would be glad to hear and respond to some if anyone enjoys this. I have the Intro and three chapters done, so after that, I'll start improvements if given the advice. Thanks all.