New Phoenix Incident

UNSC
Commander: Fleet Admiral Terrance Hood

298? Orbital Super MAC Cannon Platforms.

Spartan-104, Spartan-058, Spartan-087, and Spartan-043 on ground ops.

Spartan-117 on Forerunner Ship.

Numerous UNSC ships.

The Covenant
Commander: The High Prophet of Truth

The Forerunner Ship.

Numerous unknown ships.

Timeline
This fight, ambiguously referred to as "The Second Battle of Earth" is the follow up to the Covenant's botched first invasion, The First Battle of Earth. It is only seen as a brief cutscene where the Forerunner ship exits slipspace and the Master Chief declares that he is going to "finish the fight." UNSC and Covenant forces appear heavily engaged, and as far as we know, the battle is not over yet. Therefore numbers and details are impossible to currently determine. However it's apparent that Cairo Station is still intact and Lord Hood is still in command of the UNSC forces. Perhaps this battle will be brought into more light when Halo 3 is developed (though the trailer shows that Earth fell to the Covenant).

UNSC

 * Cairo Station
 * Various Marathon-class cruisers

Covenant

 * Forerunner Ship
 * Various CCS-class Battle cruisers

UNSC

 * Fleet Admiral Sir Terrance Hood
 * Spartan-117

Covenant

 * High Prophet of Truth

Other

 * Earth
 * The First Battle of Earth