Master Wars
From Transformers Wiki
In the year 2011, the Transformers finally moved their eternal war off of Earth, battling instead in a series of space-based exchanges between the Autobots' headquarters on Athenia and the Decepticons' base on Chaar. The super-science of the planet Master forms the backbone of the conflict, which consequently became known as the Master Wars.
Fiction
Japanese cartoon continuity
Following their final defeat in a battle at the North Pole, the Decepticons under the command of Scorponok finally retreated from the Earth once and for all. The Final Showdown on Earth (Part 2) They were pursued into space by the Autobot army led by Fortress, and there, their battles continued, supported and intensified by the advanced technology developed by the Transformers on planet Master. It was not long before the Autobots achieved superiority in the conflict, since the Decepticons were without strong battlefield leadership after the destruction of Scorponok's Transtector. The Headmasters Now!!
Both the Autobots' and Decepticons' space bridge network had previously been destroyed, Daniel Faces His Biggest Crisis Ever!! and so access to Earth was almost completely cut off, keeping the planet comparatively safe. Just to be safe, the Autobot Trainbots—some of the few Autobots able to traverse interstellar distances under their own power—periodically checked in on the Earth, observing that a team of Autobot Pretenders led by Metalhawk were active in secret on the planet (and, in actuality, had been for thousands of years). Unfortunately, a great evil was secretly brewing on Earth without the Autobots' knowledge: the mysterious alien energy-entity, Devil Z, had awoken from a slumber beneath the Earth's oceans, and set about taking control of the Decepticons for his own wicked ends. The Headmasters Now!! Abducting and brainwashing Scorponok, he placed the Decepticon leader, and by extension the whole army, under his control. Shūta and Grand's Masterforce Super-Secrets!
The Autobots became aware of Devil Z's shadowy movements thanks to the former Decepticon Sixshot, who learned of his existence and sent a cryptic warning to his former enemies. At the same time, the Trainbots were returning from their latest voyage to Earth, and caught the first glimpse of Scorponok's new Transtector, "BlackZarak", hidden in a meteor cluster. With the Decepticons now active in space again and the Autobots unable to spare any of their fighting forces to deal with the problems arising on Earth, six Transtectors and six sets of Master-Braces were dispatched to Metalhawk by Chromedome, so that he could bestow them upon humans and allow them to become Transformers to defend their own planet against the Decepticon threat. The Headmasters Now!!
As the Masterforce War between Metalhawk's Autobots and the Decepticons under Devil Z's command on Earth erupted, the Master Wars themselves continued to rage in space, Birth! Headmaster Jrs though BlackZarak and Grand Maximus eventually broke off to join the fighting on Earth. Expose the Decepticons' Dark Trap! Presumably, the conflict was considered to have come to an end when Devil Z was destroyed at the conclusion of the Masterforce War, and the Decepticons were subsequently reorganized under Deathsaurus and began spreading their influence to other planets. The Brave Hero of the Universe - Star Saber
