You don't have the authority, Sir.
There have been thousands of people asking what Halo: Reach will be about.
I have read all the books and played all the games, and i have put my conclusions together.
If you remember correctly, Captain Keys says "Cortana, all i need to know is did we lose them" and Cortana Replies "I think we both know the answer to that." Well what does this mean? where were they coming from, why did they run?
In the Halo: Rach Teaser Trailer, it says From the Beginning, you know the end. This does mean we will be playing a story before Halo Combat Evolved.
From the Books of Halo: Reach, it seems like there in a battle with many other spartans, marines, ODST's (and a rumour of Buck being in the game,) it seems that this battle was big.
In Halo 3: ODST Buck Says "Its like Reach all over AGAIN" this shows that Buck, Dare and his team were fighting in reach, but we cant be sure if the Rookie was.
During the battle of reach, the UNSC start to lose, they are told to retreat and return to there ships, but its too late. The covenant had secretly planted bombs on the ground, so as the UNSC were retreating, they detonated them. The whole of the UNSC is scattered across the Universe including the pillar of autumn, but as the ship is carrying the only survivor of Spartans, it is chased. Thats Why Keys says "Did we lose them"
It isnt clear on how the other spartans were killed, i think it was from the bombs, but then how did Chief survive, he was still hibernating in the ship, he was protected.
So in Halo: Reach, you will be playing as other spartans and may see chief, or even the pillar of autumn.
EDIT - Halo: Reach has nothing to do with a halo, they didnt even know about a Halo, it became halo because a new war began over the ring, if they didnt find the halo, halo wouldnt be called halo, it would be Reach: Combat evolved, Reach 2, Reach 3, Reach ODST and The Fall of reach (Halo: Reach)
this is what i have put together, i have missed a few pieces out as they are major spoilers and i wish i didnt know them.
NOTE: THIS MAY NOT BE EXTREMELY ACCURATE.