New Phoenix Incident

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 is apparent that Cairo Station is still intact and Fleet Admiral Hood is still in command of all UNSCDF forces. The story of Halo 3 surrounds this battle.

Prior To Second Battle of Earth
Ghosts of Onyx expanded on the battle, which was stated to have been going on for two weeks during the Master Chief's disappearance. Spartans 104, 043, and 058 had been conducting commando strikes against the Covenant. These operations included numerous operations in orbit around Earth, stopping a Covenant invasion on Mount Erebus in Antarctica with a HAVOK tactical nuke, and stopping Covenant forces on the sea floor off of the Yucatan peninsula. Following these ops, they were deployed to the Centennial Orbital Elevator (also known as Tallo Negro del Maiz) in Havana, Cuba to stop Covenant forces that had gathered around the orbital elevator. They then stopped the Covenant forces, who had captured multiple nuclear warheads, before they could ascend the orbital elevator. The Spartans then ascended the orbital elevator themselves, and in the process destroyed one of the Covenant capital ships at the top of the elevator. The three Spartans then leave the battle in another Covenant capital ship called the Bloodied Spirit on orders by Fleet Admiral Hood to aid Dr. Catherine Halsey on Onyx.

UNSC

 * Fleet Admiral Sir Terrence Hood
 * Commander Miranda Keyes
 * Commander Richard Lash
 * Lieutenant Junior Grade SPARTAN Fred-104
 * Master Chief Petty Officer SPARTAN John-117
 * Sergeant Major Avery J. Johnson
 * Gunnery Sergeant Pete Stacker
 * Petty Officer Second Class SPARTAN Linda-058
 * Petty Officer Second Class SPARTAN Will-043

Covenant Separatists
{Elites Grunts Hunters
 * Arbiter
 * SpecOps Commander Rtas 'Vadumee

Covenant Loyalists
Brutes Jackles Drones
 * High Prophet of Truth

Flood

 * Gravemind