From Halopedia, the Halo wiki
He took part in the Battle of Sigma Octanus IV on July 18, 2552. However, after the encounter with the Hunters, which killed most of the unit including CO Second Lieutenant McCasky, and fleeing a fight with a small unit of Grunts, Walker was frozen with fear and had a mental breakdown. Walker's breakdown would have cost his squad their lives had Private Cochran not taken the initiative to shoot down a couple of pursuing Banshees. Walker's breakdown slowly vanished upon returning to Alpha Headquarters, near the city Cote d'Azur, and Harland eventually regained trust of Walker and allowed him to fight again. He and his team were later evacuated when the SPARTAN-IIs destroyed all the Covenant forces using a HAVOK nuclear warhead.
In the comic adaptation, Halo: Fall of Reach, Cochran is not present in the Warthog. Instead, Walker is able to regain his faculties and shoot down a lone Banshee.
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