The Siege of Coruscant was a significant engagement in space that occurred in 9 ABY during the era of the Thrawn campaign. Grand Admiral Thrawn of the Empire, facing a larger New Republic defense fleet protecting their capital world of Coruscant, ingeniously utilized cloaking technology to create a siege weapon. His strategy involved equipping twenty-two asteroids with cloaking devices, which were then transported aboard several of his Star Destroyers.
The Dark Jedi Joruus C'baoth, a clone of Jorus C'baoth, pressured Imperial Grand Admiral Thrawn, the supreme commander of Imperial forces at the time, to initiate a planetary attack against Coruscant. C'baoth's aim was to lure the Solo twins to him after Molo Himron's Imperial commando squad failed to capture them. This plan aligned with Thrawn's own strategic objectives, as he had been planning an offensive into the core territories of the New Republic. Despite C'baoth's skepticism about Thrawn's compliance and his temporary seizure of the Chimaera and its crew, Thrawn consented to prepare for the assault on Coruscant. He did so after replacing the Chimaera's crew, who had been incapacitated by C'baoth, with a caretaker crew of five hundred men led by Captain Dorja.
Thrawn agreed to ready his fleet for an assault on Coruscant once his special cargo of asteroids, sourced from Bilbringi, had been loaded onto his ships. Despite having been the instigator of the attack, C'baoth requested to be returned to the Mount Tantiss fortress on Wayland while the Imperial fleet was mobilizing for the Coruscant offensive. The Draklor, commanded by Freja Covell, was detached from the main fleet to transport C'baoth to Wayland. Shortly after C'baoth's departure, Thrawn's fleet commenced its journey toward Coruscant.
Before the siege commenced, Grand Admiral Thrawn attacked the world of Mrisst as a diversionary tactic to draw the Coruscant sector fleet away from Coruscant. This maneuver left the New Republic capital vulnerable when Thrawn initiated the Siege of Coruscant.
The battle unfolded with the emergence of ten Interdictor cruisers and eight Katana Dreadnaughts from hyperspace near Coruscant, swiftly followed by six Imperial-class Star Destroyers. These Imperial vessels were met with resistance from Coruscant's defense fleet and a pair of Golan III platforms. However, Admiral Hiram Drayson of the New Republic and his navy, including the Home Guard Fleet, were significantly outmaneuvered by Grand Admiral Thrawn's superior tactical prowess. Thrawn, directing the Imperial forces from the bridge of his flagship, the Chimaera, with the assistance of his Captain, Gilad Pellaeon, consistently outwitted Drayson, granting the Imperials an initial advantage. General Garm Bel Iblis, observing the engagement, recognized Thrawn's signature tactic of employing Interdictors to facilitate precision microjumps during combat. Nevertheless, he was unable to alert Drayson before two Victory-class Star Destroyers executed a surprise attack on one of the Golan platforms at close range. V-150 Planet Defender ion cannons were ineffectively fired at Thrawn's ship, with as much chance of hitting the Golan III battle platforms as they did Thrawn's fleet. Although DP20 Corellian gunships, X-wings and B-wings engaged the Empire, they could not stop Thrawn's plans. Shortly afterwards, Bel Iblis assumed command of Coruscant's forces at the urging of the Head of State of the New Republic, Mon Mothma, as Bel Iblis had previously refused to take command until being approved by Mon Mothma, so as to make sure she trusted him. Bel Iblis withdrew the ships defending the Golan platforms, understanding their resilience better than Drayson. This effectively presented Thrawn with two options: to remain in his current position and engage the battle stations, or to pursue the defenders, making himself vulnerable to ground-based weaponry.
At this juncture, Thrawn instructed the Death's Head and other vessels under his command to deploy asteroids into low orbit in coordination with the Chimaera, before ordering a retreat into hyperspace, effectively creating an invisible hazard zone around the galactic capital. In addition to the twenty-two actual asteroids, Thrawn initiated 265 false launches. Because the cloaked asteroids were undetectable by Coruscant's sensors, the Republic was unable to differentiate between genuine and false asteroid launches. The first asteroid collided with the escort frigate Evanrue. General Bel Iblis, correctly deducing the nature of the cloaked objects, commanded the Dreadnaught Harrier to target the asteroid's projected position with its ion cannons. Thrawn allowed the asteroid's cloak to be deactivated, confirming its identity, before destroying it with turbolaser fire to prevent further analysis. Following the successful deployment of the cloaked asteroids, the Imperial forces retreated.
Although the Imperial forces had withdrawn, the New Republic now faced significant challenges. They were unable to lower the planetary energy shield due to the risk of asteroid impacts on the densely populated cityscape, and they could not permit space traffic near the planet for fear of collisions. Initially, Rieekan and Bel Iblis, among others, implemented a strategy involving the release of a dust cloud from the first transport, followed by the use of the Harrier's negative ion beams to focus it. This dust cloud would reveal any asteroids passing through it, allowing the defenders to destroy them. While effective initially, this method was deemed too time-consuming and was abandoned in favor of acquiring a crystal gravfield trap, a sensor capable of detecting the cloaked asteroids in orbit around Coruscant. Although Republic ships successfully destroyed all of the asteroids by firing blindly into space, they had no fail-safe way of detecting the asteroids, or of knowing how many asteroids were originally launched and how many remained floating in orbit. Consequently, the galactic capital was effectively blockaded until a means of detecting and neutralizing the phantom asteroids could be developed.
These events culminated in the Battle of Bilbringi, where the New Republic attempted to seize a crystal gravfield trap to locate any remaining asteroids orbiting Coruscant. Following Thrawn's assassination during the battle, the New Republic successfully obtained the gravfield trap and was finally able to lift the quarantine.