Battle of Skopje
From Halopedia, the Halo wiki
During the battle, Orbital Drop Shock Troopers were sent to protect the headquarters of a shipbuilding corporation during the evacuation of their shipyards, under the justification that the machines, tools, and personnel that could be saved would be relocated to Reach to continue building parts for the war effort. As such, the ODSTs were told that they had to hold the perimeter at all costs. The Covenant deployed Unggoy against the humans in waves, then followed these waves with Kig-Yar Snipers, then Jiralhanae shock troops, and finally Sangheili warriors. The Covenant eventually overran the ODSTs.
Gage Yevgenny and another ODST were rescued from approaching Covenant forces by a strange soldier clad in gray-green armor. The soldier led the survivors to safety while engaging smaller aliens with impunity and meeting Sangheili and Jiralhanae on equal terms. The ODSTs were led to a stone castle that was being used as the Marines' command center. The Covenant's low-level energy weapons couldn't vaporize or burn the stone, just melt it slightly, making it stronger. Yevgenny soon learned that the soldier was a SPARTAN, part of a top-secret project that the Office of Naval Intelligence had recently revealed to the public.
Some time later, the UNSC abandoned the planet, and the Covenant glassed it.
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