Our big green style, cannot be defeated!
-Sgt. A.J. Johnson - Halo 2 - Metropolis
I would pick up as many marines on the ground as I can, & ask Cortana to find the best jump coordinates to follow. Leaving & sacrificing all civilian lives on Reach, since honestly none of them are going to survive. If I did that, Cortana would end up bringing a fleet of UNSC ships to Installation 04, where we can pull guerrilla tactics & destroy Halo much sooner. We would also have a hand full of Spartans or more, since we evacuated most of the UNSC forces on the ground at Reach (We "couldn't find" Noble team, so, oh well). In the end, the events of Ghosts of Onyx would probably still happen, only sooner since the UNSC ditched Reach.
So during the destruction of Halo, all UNSC ships get off the ground before it explodes. The Covenant ships have all landed but a few were in orbit. So most of them are destroyed, evening the battlefield & we win a space battle. Now, we follow the Cole Protocol back to Earth, with Captain Keyes & the Chief heading back to Reach to save Dr. Halsey, which they do. So they sneak behind enemy forces & rescue her & the spartans assisting her. We follow the Cole Protocol back to Earth. Now, with a few spartans, (at least 30) we easily fend off Covenant forces on the ground, & send them all to fight (starting Halo 2), but they don't go with the Chief to Installation 05. So when Halo 3 starts, we still have all the spartans (maybe one or two dies...) & we easily fend off the ground forces of brutes & flood. They all go to the Ark, & wreck the moment they touch the ground (Imagine every mission on Halo 3, with 29 other spartans right behind you). So Sgt. Johnson & Miranda Keyes would survive for sure, & Chief would go to rescue Cortana by himself, since we wont risk other Spartans dying. So now that he does it, we simply nuke the control room, instantly killing 343 Guilty Spark & the control room, & the end mission would never happen. So basically, it's a happy ending, except for the fact that we killed the Arbiter. So, we never made friends with the elites. Lol
Edit: I'm an Admiral
Edit: The elites possibly come back for revenge...Since they came back after the war, to destroy the rest of the Covenant (The Blood Covenant)
[Edited on 06.26.2011 11:40 PM PDT]