Assault on Seelos

In the fourth year before the Battle of Yavin, the Galactic Empire launched an assault against rebel and clone forces on the planet Seelos.

Prelude
In an attempt to find a planet to host the rebellion's new base, Ahsoka Tano suggested the Ghost's crew to search her fellow during the Clone Wars Captain Rex in whose hands there was a map of useful locations. The Lothal rebels traveled to Seelos, the planet on which the clone had probably retired after the war. There Kanan Jarrus, Ezra Bridger, Sabine Wren and Garazeb Orrelios reached the surface on the Phantom, leaving Hera Syndulla on the Ghost flying over the planet.

The rebels found the clone riding a modified All Terrain Tactical Enforcer, a Clone Wars-era walker, together with two comrades, Commanders Wolffe and Gregor. After some adventures and debates, the group seemed to get along, but Wolffe who was unhappy with the rebels and Ahsoka, alerted the Galactic Empire. Despite realizing his folly, the rebels' position was now exposed to the Empire. Shortly later, an Imperial probe droid discovered the Phantom and inflicted damage on the starship before being destroyed by Rex.

Tracking Wolffe's signal, the Imperial Security Bureau Agent Kallus reached Seelos aboard the Star Destroyer Relentless. Shortly later, Agent Kallus contacted the clones by intercom and demanded that they surrender the rebels. However, Rex rejected Kallus' demands and he and his fellow clones resolved that they would fight in order to allow the rebels to escape offworld. Before leaving, Rex handed Ezra a list of Old Republic military bases in the Outer Rim Territories and Mandalore.

The Battle
Instead of an orbital bombardment, Kallus decided to deploy three AT-AT walkers to destroy enemy forces, represented just by the clones' AT-TE. To gain an advantage, Wolffe commanded the walker into an approaching sandstorm, causing all the vehicles to be blinded. However, the Force-sensitive rebels Kanan and Ezra were able to use their Force to guide the clone crew through the sandstorm.

The AT-ATs and AT-TE attempted to outmaneuver each other until Kallus were able to surround the clone walker with an enveloping manuever. Rex soon sent Ezra to use the Force to destroy one of the AT-ATs. This took the Imperials by surprise since they not aware that they were so close with the enemies. Kanan pointed Ezra in the right direction, and the young rebel centered one an AT-AT with the AT-TE's main gun. Then the clone walker left the sandstorm, pursued by the remaining two walkers.

At this point in the battle, Admiral Kassius Konstantine was ordered by Lord Vader to meet with his shuttle, forcing him to abandon the battle. The Ghost was finally able to descend to Seelos without having to contend with the Imperial Star Destroyer. Meanwhile the rebels left the AT-TE aboard the Phantom to rendezvous with Hera and Chopper on the Ghost while Rex and the clones charged the two remaining walkers in a suicide attack.

Despite taking several hits, the clones' AT-TE was able to ram the legs of Kallus' AT-AT. However, Kallus' more powerful and taller AT-AT soon gained the advantaged over the vintage AT-TE walker. In spite of the odds against them, Rex and his fellow clones resolved to continue fighting. At the urging of Ezra, Kanan agreed to help the clones. While Sabine flew the Phantom, Kanan, Ezra, and Zeb entered the head of the second remaining AT-AT and hijacked the walker. They then used the walker's guns to attack Kallus' walker.

Kallus attempted to call for reinforcements but Admiral Konstantine was unable to render any help since he was preoccupied with hosting the Fifth Brother, another Inquisitor. With Kallus' AT-AT walker weakened, the clones' AT-TE walker was able to regain power. This allowed Rex to fire the AT-TE's laser cannon into the AT-AT walker's neck, destroying the Imperial walker. Kallus and two surviving AT-AT pilots escaped the battlefield in a speeder. Out of gratitude to the rebels, Rex along with Wolffe and Gregor saluted Ezra and Kanan.

Aftermath
While Wolffe and Gregor chose to stay behind on Seelos, Rex decided to aid the rebellion and traveled with the crew of the Ghost. The group met with the rebel flotilla, where Ahsoka Tano saw her fellow and had a brief talk with him. Since the events on Seelos, Rex has often helped the rebels in their missions, such as in the Blockade of Ibaar, in the rescue on the Imperial Interdictor, the Battle on Garel , an ambush in the Geonosis system , and the establishment of Chopper Base on the planet Atollon.

Behind the scenes
The assault on Seelos first appeared in the Star Wars Rebels episode "Relics of the Old Republic", which premiered at the New York Comic Con on October 8, 2015.