Some months ago, Tantor had defected to the Rebel Alliance. As a high-profile Imperial general, Tantor faced significant suspicion from General Tyr Taskeen, his new superior. To prove his goodwill, Tantor offered to acquire an AT-AT for the Rebellion. Taskeen agreed.
Tantor first led a mission to Ruul that freed his old mentor, Beri Tulon, from Imperial custody. Tulon gave Tantor the location of an AT-AT factory: Trasse, where he had served for one and a half years. Tantor and Tulon stole the access codes to the Trasse system from the Sounder Flats computer complex, then began the preparations for the raid on Trasse.
With Tulon's unit of Infiltrators trained to enter and seize control of AT-ATs, Taskeen, Tantor, and Tulon set off for Trasse. They were able to enter the system undetected with the stolen codes and landed a light force a short distance from the Proving Grounds. Tantor's Y-wings disabled at least one AT-AT on the Proving Grounds, clearing the way for the insertion of the Infiltrators.
Though the stolen goods returned to the base safely, their extraction was compromised by the arrival of the Star Destroyer Inquisitor over the asteroid, blocking the Rebels' escape. Its commander, Grand General Malcor Brashin, opened up communications with Tantor. Tantor ignored his old superior's taunts, instead offering him the chance to defect to the Alliance. Unsurprisingly, Brashin refused and began landing troops.
Tantor took his one chance, hammering through the Imperial forces to the nearby base, home to a planetary ion cannon. His troops rushed the base, taking control of the cannon, from which Tulon fired three shots. Those blasts were enough to disable the Inquisitor, leaving the Rebels free to escape. In the course of slicing into the cannon's computer banks, Tulon discovered increased shipments of walkers to Endor. Tantor correctly interpreted this as a sign that the Death Star II was nearly operational.