New Phoenix Incident

"Sir - finishing this fight."

- SPARTAN-117 to Fleet Admiral Hood

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 Africa fell to the Covenant).

Ghosts of Onyx expanded on the battle, which was stated to have lasted 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. Linda, along with the other Spartans, stopped the Covenant forces, who had captured multiple nuclear warheads. The Spartans stopped the Covenant, and they ascended the orbital elevator, destroyed one of the Covenant capital ships at the top of the elevator. The three SPARTANS leave the battle in another Covenant capital ship Bloodied Spirit on orders by Lord Hood to aid Dr. Catherine Halsey on Onyx.

UNSC

 * Fleet Admiral Sir Terrence Hood
 * Master Chief Petty Officer SPARTAN John-117