The Infinity War, also called the third galactic civil war was a large conflict that took place in our galaxy fought between the newly-established Federation of Free Planets, the successor of the Galactic Alliance Remnant and Dark Reach, a coalition of independent factions united by Sith Lord Darth Cyrus.
At the conclusion of the third Imperial civil war in 146 ABY, the Imperial Confederation joined the war after knocking the Fel Empire out. The Federation and the Confederation signed a brief truce for the duration of the Yavin campaign, but it came to an abrupt end during the siege of Xelos V.
A Dark Lord of the Sith, Darth Cyrus had wandered the galaxy for three centuries, experienced the rise and fall of several Sith orders during this time. On a young age, he received a premonition that he would one day rule the galaxy. In 11 ABY, he set out to do that, establishing his own Sith Order on Korriban. The group became nomadic however in an effort to stay hidden.Cyrus continued wandering the galaxy, recruiting allies for his eventual war against the galaxy, including the last Sith, Arhawâr and his future second-in-command, Darth Xaviun. After the Lost Tribe of the Sith crisis, Cryptum settled down on Korriban and slowly started expanding to nearby Sith Worlds in the Esstran sector including Ziost and Dromund Kaas.
While Cryptum remained hidden during the Sith–Imperial War and the second Imperial civil war, Darth Cyrus went on a trek through the galaxy to all remaining independent governments and corrupted them into allying him, establishing Dark Reach, and Cyrus prepared for his war on the galaxy.
Relations between the Fel Empire and the Galactic Alliance had been poor for the larger half of the century and their renewed wars between 128 ABY and 140 ABY made matters worse, despite their alliance during the latter half of the decade. At the end of the conflict, in 140 ABY, the Galactic Federation Triumvirate was established to unite the Galactic Alliance Remnant, the Empire-in-Exile and the New Jedi Order.
The triumvirate ruptured in 141 ABY, less than a year after its establishment. The Empire-in-Exile was reorganised into the Fel Empire again, under leadership of the heir Marasiah Fel. The Galactic Alliance Remnant returned to how it was before the war under leadership of admiral Gar Stazi. Despite desperate attempts, no new leader was elected in the years following. During the skirmish over Coruscant, Gar Stazi was killed by the Yevethan Protectorate. This culminated in a declaration of war between the two governments and the reformation of the Galactic Alliance Remnant into the Federation of Free Planets and the establishment of the Federation of Free Planets Senate. Chancellor Aaron Rahn was the first to be elected into office.
The enemy has us bogged down on Kashyyyk! While we try to lift the siege, they're going about conquering! Cybe, Jabiim, Boz Pity! We need to withdraw!
—Supreme Admiral Kai Ar'Lya to Chancellor Aaron Rahn.
The war abruptly began with the battle of Kashyyyk. Darth Cyrus, being well-aware of the value the planet held to the Federation, personally led a strike on Kashyyyk in an effort to draw the Federation into an extended siege, successfully. After the initial invasion and the establishment of a foothold, Darth Cyrus retreated but deployed forces to Trandosha, establishing a base of operations on Trandosha. The Trandoshans were split in two, with on one hand supporters of Darth Cyrus, and on the other hand the allies of the Federation, the Trandoshan Hegemony.
The on-going rout at Kashyyyk permitted Dark Reach to perform lightning strikes on other poorly-defended Federation-aligned planets, including Boz Pity, Jabiim and Cybe. At this point, Cybe Drives had become a very important asset to the Federation's war effort, as the Federation of Free Planets Navy was primarily comprised of Cybe products. Knowing this, Darth Cyrus sought to destroy the shipyards in an effort to cripple ship production. While relatively successful, only 60% of the shipyards could be destroyed, as with the effort of the New Jedi Order, the siege of Kashyyyk was lifted, allowing the Federation to deploy a taskforce to Cybe and driving away the Sith force.
Core Worlds Campaign
With no chance of recuperation, the Sith onslaught continued, targeting the Core Worlds and Coruscant, the capital of the Federation, in particular. The ensuing battle was a devastating defeat for the Federation, as Coruscant was razed and nearly the entire population is killed in the ongoing chaos. Despite this, the Federation's back was not broken yet, and the capital was relocated to Foron, promptly reactivating the Foronian Senate and repurposing it as the new Senate.
A vassal government was established on Coruscant, led by a collaborator from Coruscant. This vassal government, aided by Arhawâr, waged the remainder of the Core Worlds campaign against the Federation. While the initial campaign was a success, with the battles of Corulag, Anaxes and Fresia being Sith victories, the following battles were a string of defeats, and the campaign was called to a halt.
Siege of the Hapes Cluster
Concurrent to the Core Worlds campaign ran the siege of the Hapes Cluster, beginning with the fall of Hapes. While the initial attack on Hapes was led by Darth Cyrus, the remainder of the campaign was left to admirals Fiaval Harrsk and Rhys Daruun, and commander Torven Kark.
The campaign was a large success, despite Hapan rebellion, the Hapes Consortium ceased to exist, however the aftermath of the battle of Olanji resulted in the start of the Mist War, a series of hit-and-run strikes condoned by the Hapan government, supported by the Federation of Free Planets against the Hapes Protectorate.
By 148 ABY, the Mist War had effectively concluded, however.
Yavin and Xelos
In 146 ABY, the Fel Empire had been knocked out of the war by the Imperial Confederation during the concurrent third Imperial civil war, eliminating another ally of the Federation. This began a three-way war between the Federation, Dark Reach and the Confederation, leading to the inevitable incursion of Yavin. In a surprising gesture however, the Confederation and the Federation signed a truce, both governments willing to set aside their differences to destroy Dark Reach first.
The truce was however of short duration; with the following siege of Xelos V, the truce came to an abrupt end and resulted in a stand-off between Grand Admiral Arcus Haven and Supreme Admiral Kai Ar'Lya, with the former carefully securing a victory. It was after the battle that the Confederation found out that Xelos had been the result of Sith Alchemy. The planet was promptly annexed into the Confederation.
At the same time, admiral Akran Zelvden of the Confederation launched a strike on Yavin 7 and successfully destroyed the Dark Reach force protecting it, leading to the brief annexation of the planet. Yavin was abandoned shortly after.
Civil Wars and blockade
For the duration of the war, Dark Reach had kept a tight guard on their allies, the Predecessors on Isolum, the only one of Cyrus' allies he hadn't managed to corrupt but willingly joined him instead. During this time, a strong Dark Reach presence was on and over the planet. Supreme Predecessor Cron Aruís and his commanders decided to break free, and the battle of Isolum began.
The battle was a devastating defeat for Dark Reach, and led to a string of battles between the Predecessors and some of Dark Reach's vassal states in the Unknown Regions. The Chiss Ascendancy joined the Predecessors, the Varino Imperium entered a civil war, and the Aradan Sectorate was destroyed altogether.
In this time, unrest and instability had come to a climax on Naboo, resulting in the battle of Naboo. While scarce, the Federation deployed units to aid the Naboo loyalists, while Dark Reach had elected to aid the revolutionaries. A destructive battle for the Federation, Dark Reach had indirectly destroyed many of its forces planetside, and subsequently abandoned the rebels. The short-lived civil war eventually was a Federation victory, and a new royal was installed on the throne of Naboo.