In the year 4 BBY, the forces of the Galactic Empire initiated a military offensive against rebel groups and former clone trooper units stationed on the planet known as Seelos.
Ahsoka Tano, looking for a suitable world to establish a new base for Phoenix Cell, advised the Ghost's crew to locate Captain Rex, a comrade from the Clone Wars era, who possessed a map detailing strategically valuable locations. The Spectres journeyed to Seelos, the world where the clone was believed to have settled following the war. Kanan Jarrus, Ezra Bridger, Sabine Wren, and Garazeb Orrelios landed on the planet's surface aboard the Phantom, while Hera Syndulla remained aboard the Ghost, providing aerial support.
The rebel group encountered the clone commander piloting a modified All Terrain Tactical Enforcer, a walker from the Clone Wars, accompanied by two fellow veterans, Commanders Wolffe and Gregor. After a period of adjustment and disagreement, the group began to find common ground. However, Wolffe, harboring resentment towards the rebels and Ahsoka, contacted the Galactic Empire. Despite his regret, the rebels' position was now compromised. Shortly thereafter, an Imperial probe droid detected the Phantom, damaging the starship before Rex destroyed it.
Agent Kallus of the Imperial Security Bureau, tracking Wolffe's signal, arrived at Seelos aboard the Star Destroyer Relentless. Agent Kallus soon communicated with the clones via intercom, demanding the rebels' surrender. Rex, however, refused Kallus's demands, and he and his fellow clones resolved to fight to allow the rebels to escape the planet. Before their departure, Rex entrusted Ezra with a list of Old Republic military facilities located in the Outer Rim Territories and Mandalore.
Instead of initiating an orbital bombardment, Kallus opted to deploy three AT-AT walkers with the goal of eliminating the enemy forces, which consisted solely of the clones' AT-TE. Seeking an advantage, Wolffe steered the walker into an approaching sandstorm, reducing visibility for all vehicles. Nevertheless, the Force-sensitive rebels Kanan and Ezra were able to utilize their connection to the Force to guide the clone crew through the storm.
The AT-ATs and AT-TE engaged in a series of maneuvers, each attempting to gain a tactical advantage until Kallus managed to encircle the clone walker with a comprehensive pincer maneuver. Rex then instructed Ezra to employ the Force to disable one of the AT-ATs. This action caught the Imperials off guard, as they were unaware of the enemies' proximity. Kanan directed Ezra, and the young rebel focused the AT-TE's primary weapon on one of the AT-ATs. Subsequently, the clone walker emerged from the sandstorm, pursued by the remaining two walkers.
At this critical juncture, Admiral Kassius Konstantine received orders from Lord Vader to board his shuttle, compelling him to withdraw from the battle. The Ghost was finally able to land on Seelos without facing opposition from the Imperial Star Destroyer. Meanwhile, the rebels abandoned the AT-TE aboard the Phantom to meet with Hera and Chopper on the Ghost, while Rex and the clones charged the two remaining walkers in what appeared to be a suicide attack.
Despite sustaining multiple hits, the clones' AT-TE managed to collide with the legs of Kallus's AT-AT. However, Kallus's more powerful and taller AT-AT quickly gained the upper hand over the antiquated AT-TE walker. Despite the unfavorable odds, Rex and his fellow clones were determined to continue fighting. Prompted by Ezra, Kanan agreed to assist the clones. While Sabine piloted the Phantom, Kanan, Ezra, and Zeb infiltrated the head of the second remaining AT-AT and hijacked the walker. They then turned the walker's weapons against Kallus's walker.
Kallus attempted to request reinforcements, but Admiral Konstantine was unable to provide assistance as he was occupied with hosting the Fifth Brother, another member of the Inquisitorius. With Kallus's AT-AT walker weakened, the clones' AT-TE walker was able to restore its power. This enabled 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 fled the battlefield in a speeder. In gratitude to the rebels, Rex, along with Wolffe and Gregor, saluted Ezra and Kanan.
While Wolffe and Gregor decided to remain on Seelos, Rex chose to join Phoenix Squadron and traveled with the crew of the Ghost. The group rendezvoused with the rebel flotilla, where Ahsoka Tano reunited with her old friend and engaged in a brief conversation with him. Following the events on Seelos, Rex frequently assisted the rebels in their missions, including the Blockade of Ibaar, the rescue on the Imperial Interdictor, the Battle of Garel, an ambush in the Geonosis system, and the establishment of Chopper Base on the planet Atollon.
Wolffe and Gregor subsequently repurposed the last AT-AT into their new residence, Joopa Base. The battle also definitively proved that clone troopers were superior to the soldiers who had replaced them. Seelos once again became a battleground in the early stages of the Galactic Civil War when another former clone trooper—Wildfire, who had previously served under Wolffe during the Clone Wars—was dispatched by the rebellion on a mission to retrieve intelligence from Joopa Base.
The assault on Seelos was first depicted in the Star Wars Rebels episode "Relics of the Old Republic," which premiered at the New York Comic Con on October 8, 2015.