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When Agent Kallus and the Empire tracked the Ghost crew to Seelos, they brought with them a new kind of weapon: the AT-AT. Massive and powerful, the rebels’ clone trooper friends were impressed with the four-legged tanks’ advances in engineering, armor, and firepower. Their modified AT-TE was no match.
Three of the transports were dispatched in total, chasing the rebels into a blinding sandstorm. Through the Force, Kanan the Jedi was able to “see” the walkers and advised the clones on where and when to strike. It proved to be a successful strategy, as a direct hit to the neck of the nearest AT-AT sent the vehicle toppling. Once out of the sandstorm, the battle continued. Zeb, Kanan, and Ezra eventually commandeered one of the walkers, helping Captain Rex to destroy the only remaining AT-AT, and Kallus ordered a hasty retreat.
AT-ATs drove the Empire’s assault on the Rebel base during the Battle of Hoth. The gigantic combat vehicles were air-dropped onto the snowy world, where they marched toward the command center and were met by Rebel defenses. Snowspeeders first attempted to blast the AT-ATs, but the walkers' armor proved too strong. Commander Luke Skywalker ordered his squadron to use harpoons and tow cables, tying the legs of the hulking transports. Though this tactic proved effective at toppling a walker, the Rebels were ultimately unable to stop the Empire’s advance. General Veers, leading the surface attack, targeted and destroyed the Rebel base’s main power generator. Imperial forces, along with Darth Vader, entered the base -- though most Rebels had been able to escape.
AT-ATs were used by the Empire as transports on Endor, the forest world that was home to the Death Star II’s shield generator. Due to the dense forests and brush, the smaller, more agile AT-STs were primarily used during the Battle of Endor.