In the year 10 ABY, the scattered forces of the Galactic Empire initiated a sweeping offensive under the direction of the resurrected Emperor Palpatine. This offensive quickly devolved into the Imperial Mutiny, and was subsequently followed by Operation Shadow Hand.
Following the Battle of Endor, both the Galactic Empire and the Alliance to Restore the Republic believed Emperor Palpatine had perished in the destruction of the Second Death Star. However, his essence survived and traveled to Byss, an Imperial stronghold located in the Deep Core. There, he underwent a prolonged recovery as he was reborn into a clone body. Meanwhile, the New Republic advanced from Endor, gradually weakening the Galactic Empire, which was plagued by warlordism. By 7 ABY, the New Republic had successfully captured Coruscant.
Leading up to the Thrawn campaign in 9 ABY, Palpatine's clone secretly amassed military resources in the Deep Core for a future attack. Imperial Admiral Feyet Kiez deserted Anaxes with the Star Dreadnought Whelm on Palpatine's orders, keeping his destination secret even from his second-in-command, Osted Wermis. The Imperial warlord-turned-pirate Inos Fonada vanished along the Corellian Trade Spine with his fleet. The leading scientists from an Imperial research facility on Kelada disappeared with all their weapons blueprints. And after the seizure of Kuat, the entire senior design team from Kuat Drive Yards fled into the Deep Core aboard the partially completed dreadnought Eclipse.
The New Republic largely dismissed these reports during the near-catastrophe of Grand Admiral Thrawn's final campaign. In its wake, they reasserted their dominance with incursions into the New Territories, seizing the Imperial Ruling Council's capital of Orinda and attacking Prince-Admiral Delak Krennel's Ciutric Hegemony in the Hegemony theater. Krennel was targeted due to his murder of Grand Vizier Sate Pestage years prior, and his continued support of his reign through violent purges and the oppression of non-humans. This choice was controversial, as Krennel had remained relatively inactive since his rise to power, not venturing beyond the Hegemony. Aided by Ysanne Isard's clone, Krennel cultivated this image by denouncing superweapons, apologizing for Alderaan's destruction, admitting the Empire's wrongdoings, and offering refuge to Humans displaced by the New Republic's policies that reversed Imperial Human High Culture. As this policy continued, they were approached by numerous refugee groups and former Imperials, including Alderaanian refugees. They also communicated with Gilad Pellaeon about bringing Thrawn's fleet into the Hegemony. However, the situation changed with the discovery of the plans for the Pulsar Station, a Death Star-like superweapon. These plans were actually false, planted by the real Ysanne Isard, who assisted a New Republic force led by Rogue Squadron in liberating the Hegemony and eliminating Krennel.
Simultaneously, Palpatine's agents reached out to the Deep Core warlords, Blitzer Harrsk of Zero Command, Treuten Teradoc, and Sander Delvardus. The remnants of the Loyalist Imperial Ruling Council also recognized the reborn Emperor, as did the otherwise-isolationist Pentastar Alignment under Ardus Kaine. While the factions within Palpatine's alliance largely despised each other, they were all motivated by fear of the Emperor's retribution and the desire to share in the spoils of a successful campaign.
With Gial Ackbar and the New Republic Defense Fleet occupied far away in the Outer Rim Territories pursuing Krennel, the Core Worlds were vulnerable, and Palpatine seized this opportunity to attack. Three massive Imperial task forces emerged from the Deep Core, centered around four Executor-class dreadnoughts (including the missing Whelm), five Mandator III-class dreadnoughts, and three Vengeance-class dreadnoughts. The New Republic Navy, which had long prioritized smaller cruiser and carrier-based snubfighter operations, found itself unable to effectively counter the immense Star Dreadnoughts. The Deep Core fleets conquered Kaikielius and Alsakan. Simultaneously, High Admiral Treuten Teradoc attacked Corellia.
The New Republic hastily assembled a defense at Metellos, but its fleets were caught unprepared. Fleets from Imperial Space in the New Territories descended upon the Core from the north, conquering Anaxes and Borleias. Meanwhile, Sander Delvardus obliterated the fleet at Metellos, and the Ruling Council's forces advanced through the Borderland Regions, capturing Contruum and Columex.
Imperial fleets converged on Coruscant and began a devastating bombardment of the planetary shields. Facing inevitable defeat, Mon Mothma ordered the planet's evacuation, which was overseen by Garm Bel Iblis. The New Republic resorted to guerrilla warfare within Galactic City, but ultimately lost the battle. General Alix Balan then led an Imperial victory parade through Coruscant's boulevards.
The New Republic's government reestablished itself on Da Soocha V in Hutt Space. They considered the Empire's swift advance unsustainable and awaited an opportunity to counterattack. However, the newly unified Empire quickly fragmented into a brutal six-way civil war between the Ruling Council, the Moffs, the Fleet commanders, the Inquisitorius, COMPNOR, and the Imperial Security Bureau, known as the Imperial Mutiny. Coruscant was left as a sparsely populated wasteland. The cloned Emperor Palpatine did nothing to stop this internal conflict, viewing it as a way of "culling the weak." Instead, he used the situation to publicly reveal himself and deploy his World Devastator superweapons to Dac as part of Operation Shadow Hand.
The New Republic's poor performance against Imperial Star Dreadnoughts spurred the development of their own dreadnought-like vessels, the Mediator-class battle cruiser and the Viscount-class Star Defender, as part of the New Class Modernization Program following Operation Shadow Hand.