All Terrain Armored Transport

This model was used against the early rebellion. This variant featured two heavy laser cannon turrets, four anti-personnel blasters, and a dorsal twin laser turret at the rear.

This model was used during the Galactic Civil War. This variant was shorter than the early model, and featured dual Piperii-Cerlurn R-90C medium blasters located on the sides or "temples," and a pair of Taim & Bak MS-1 fire-linked heavy laser cannons mounted on the "chin."

This was a modified version of the Galactic Civil War AT-AT: the variant featured, among other things, thicker and darker armor, heavier laser cannons, and an elite crew of pilots.

This was an updated version of the AT-AT used by the First Order.

The AT-AT walker was developed during the Clone Wars and used in the Imperial Army, the ground forces of the Galactic Empire. It was the successor of the All Terrain Tactical Enforcer that also saw use during the Clone Wars, although it constituted a far more imposing example of walker technology. Heavily armored and awe-inducing, the AT-AT positioned itself as the standard of Imperial firepower on the battlefield.

Many AT-ATs were built in the factories of the planet Balmorra. In 14 BBY, the Empire used AT-ATs against the Partisans led by Saw Gerrera on Kashyyyk during an attack on an imperial refinery. During the battle, the Padawan Jedi Cal Kestis stole an AT-AT and used it to provide aid to Partisans. Kestis used the walker's firepower to eliminate most of the Imperial troops, but the AT-AT ended up falling.

In 4 BBY, the Imperial Security Bureau Agent Kallus led three early AT-AT walkers on an assault on the desert planet of Seelos against the Spectres and three retired clone officers aboard an AT-TE.

In 3 BBY, AT-ATs were again used against the Spectres, when Princess Leia Organa arrived on the planet to give the Ghosts crew three Sphyrna-class Hammerhead corvettes. Two walkers were deployed to patrol the transports' landing platform, which were formally on Lothal to offer supplies to citizens impacted by the rebel efforts. When the rebels tried to steal the corvettes, the Ghost started firing on the AT-ATs. Four proton torpedos were not powerful enough to destroy one of the walkers, and it was necessary for Kanan Jarrus to use his lightsaber to cut its legs, toppling it. The second one was knocked down by the engines of the last Hammerhead, piloted by Ryder Azadi.

In 2 BBY, Lieutenant Yogar Lyste deployed two AT-AT walkers, Walker 271 and Walker 414, to combat an attack on the east gate of the Imperial Armory Complex by members of the Lothal resistance group led by Ryder Azadi. The rebels Kanan, Ezra Bridger, and Chopper escaped in the stolen AT-DP Walker 216. However, Grand Admiral Thrawn realized that the rebels had hijacked the AT-DP walker and ordered his AT-ATs to crush them. One of the AT-AT walkers proceeded to crush the AT-DP under its heavy body but the rebels managed to use their lightsabers to cut their way aboard. Morad Sumar also fired a rocket which hit one of the walker's legs. After knocking out the AT-AT pilots, the two climbed out and escaped with Ryder's cell into the wilderness.

Thrawn then deployed four of these during the battle on Atollon. Jedi survivor Kanan Jarrus then destroyed one with his lightsaber. The Bendu then destroyed two of the AT-AT walkers. However, the last one shot Bendu down from the sky.

Around the time of the Battle of Yavin, a new AT-AT model was introduced. At this time, the Alliance to Restore the Republic stole a number of AT-ATs and provided them to Saponza's Gang.

During the Rebel assault on the Imperial weapons factory moon of Cymoon 1, a group of Rebels led by Han Solo and Leia Organa stole an AT-AT from Weapons Factory Alpha. They used the walker to escape from the factory after they sabotaged its main power core to explode. During their escape, Solo fired a shot that nearly killed the Sith Lord Darth Vader, using all the AT-AT's powerful armaments. Soon after, Vader managed to overtake the Rebels in their escape and cut down the massive walker's back left leg. The AT-AT fell tremendously and its passengers barely escaped the wreckage before fleeing Cymoon 1 entirely for the safety of the Alliance Fleet.

During the Battle of Hoth, General Maximilian Veers led a squadron of at least five AT-ATs on the icy fields of Hoth to knock out the shield generator that protected Echo Base, the Rebel headquarters. The triumph of Veers' squadron allowed the Imperials to invade Echo Base and rout the Rebels, striking a major blow in the war. However, the battle also revealed that the AT-ATs could be toppled by a cable wrapped around their legs. Although that weakness had been used effectively by the Alliance on Hoth, the Empire continued to use these mechanical behemoths in the later stages of the war.

Several AT-ATs were deployed by Admiral Kalaxo during the Battle of Ab Dalis. A number of AT-ATs marched as part of an Imperial Unity Day parade on the planet Bar'leth in the Core Worlds, where the Empire was caught by surprise by a rebel attack. Caught totally offguard, all AT-ATs and other mechanical marchers were destroyed by the time the rebels withdrew in an attack that was transmitted to the entire galaxy, proving the rebellion had survived the Battle of Hoth. During the occupation of Endor, at least one AT-AT was present on Endor prior to the Battle of Endor.

Shortly after the Battle of Endor, AT-ATs were present for the Battle of Cawa City. The rogue agents of Inferno Squad Iden Versio and Del Meeko used an AT-AT in order to escape Vardos, during Operation: Cinder. In 5 ABY, at least one AT-AT walker was stationed at Camp Sardo on the planet Kashyyyk. During the Liberation of Kashyyyk, several Wookiee guerrilla fighters, including Kirratha, hijacked the AT-AT walker and threw out the crew. Later, AT-ATs were deployed against New Republic forces during the Battle of Jakku.

Decades after the war, a young scavenger named Rey used one of the downed AT-ATs, designated Hellhound Two, as her home on Jakku.

The First Order used an updated version of the AT-AT in its army branch. The walkers were deployed under the command of Kylo Ren, the master of the Knights of Ren and grandson of Darth Vader, during the First Order's brief struggle against the Benathy species. Later, during the Battle of Crait, these AT-ATs escorted the newer AT-M6s. This model of AT-AT walker was also stored aboard Commander Pyre's Resurgent-class Star Destroyer, though the warship was ultimately destroyed during the rescue of Tamara Ryvora.

The AT-ATs in The Empire Strikes Back and the brief appearance in Return of the Jedi were filmed via stop motion.

Initially, the Empire was to attack the Rebel base on Hoth using tanks. Joe Johnston created many designs of these vehicles, one of which was the basis for the HAVw A6 Juggernaut that appeared in Star Wars: Episode III Revenge of the Sith and the HAVw A5 Juggernaut in Star Wars Legends, and the tanks themselves were to be portrayed by remote-controlled vehicles. However, Dennis Muren and Joe Johnston convinced George Lucas to use stop motion instead, which eventually led to the abandonment of the tank design. In the DVD commentary to The Empire Strikes Back, George Lucas explains that he got the idea of the AT-AT from the massive tripods from H.G. Wells' 1898 novel The War of the Worlds. Though it is popularly believed that dock machinery was the inspiration for the AT-AT, Phil Tippett has said otherwise, recalling that it was based on the prehistoric mammal Paraceratherium.

The AT-ATs featured in Star Wars Rebels are different models from the ones in the Star Wars films, inspired by the early concept art drawn by Ralph McQuarrie during the production of The Empire Strikes Back.

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