Second Battle of Mombasa
From Halopedia, the Halo wiki
It appears, from hints in the Believe Ad and Museum, that the battle for Mombasa did not end with its destruction. Later, during the Second Battle of Earth Human and Covenant forces clashed again in its ruins, both forces surging foward to capture it and whatever prize or tactical value it may of held. Elements from both the 506th Tank Regiment and the 77th Marine Regiment battled furiously for it, with snipers from both sides taking a heavy toll upon the enemy. Ultimatly, the battle lasted Seven days, with the UNSC forces in a desperate situation. With ammo becoming scarce and all hope seeming lost, that is when Master Chief stepped into the battle.
Battle for the Landing Zone
The Second Battle of Mombasa was started on the outskirts of the battlezone, where UNSC and Covenant forces engaged each other bitterly for control of a Landing Zone. The UNSC never gained full control of it, always being pushed back by the Covenant when ever they seemed to secure it and Visa Versa.
The Northern Bridge Falls
Despite a valiant defence by the Marines, there was no way they could hold the bridge for any longer. With Brute forces surging over it and hitting the Marine's vulnerable flanks, there was only one choice. At 14:00 the order came through the deostroy the bridge, so to force the enemy forces down into the Valley where UNSC Trenches lay to slaughter the enemy with fire from elevated positions. However, this was not to be as the Covenant forces had already began to surge down there with the destruction of the Bridge.
The Hill
The most ferocious and bloody bottleneck in the battle, the UNSC forces sent to defend it took 2/3 casualties. After second bridge was destroyed, the treches at the foot of here, a last ditch defensive measure, came under heavy attack, garnering heavy Marine casualties.
Snipers
Snipers from both sides battle for control, with UNSC Snipers fighting on one end of the destroyed southern bridge and Covenant Jackal snipers halting Marines outside the Landing Zone before being sent into a rout by UNSC infatry and tanks.
The 506th arrive
Tasked with securing a road to the southern bridge span, the 506th plowed foward, taking heavy casualties. Though outnumbered, they took their objective, leaving a path for the second wave of UNSC infantry to follow.
John's Gambit
It was at the top of the Hill bottleneck that John-117, aka the Master Chief, launched a gambit to retake the hill for the UNSC forces, by allowing himself to be captured by Brute forces there. Though apparently feigning defeat, John armed a plasma grenade and set events into motion that would mean victory for the UNSC, both for this Battle and the battle for Earth and seal his place as one of the greatest heroes and warriors in human history. Despite this, his fate is unknown.