Willham Burke held the position of Admiral within the Rebel Alliance's Fourth Battle Group, with operations concentrated in the Outer Rim. Subsequently, he took on the role of commander in the New Republic's Third Fleet, securing numerous victories in the immediate aftermath of the Battle of Endor.
Considered a highly skilled commander, he was greatly admired, and it was seen as a significant honor for Rebel pilots to meet him. He contributed to the training of various pilot squadrons, including Havoc Squadron, Vortex Squadron, Crimson Phoenix Squadron, as well as independent pilots from CorSec and the Smugglers' Alliance. Shortly after the Battle of Yavin, he was reportedly seen at a hotel on Corellia, specifically in Doaba Guerfel. While present there, he tasked a single individual to battle against 30 Imperial starfighters near Kessel, which was followed by an assault on the Imperial CR90 corvette known as the Star Ravager.
During 5 ABY, Admiral Ackbar temporarily delegated command of the New Republic Third Fleet to Admirals Burke and Kalback. Their mission was to isolate the forces commanded by Imperial warlord Treuten Teradoc from the Core Worlds. Leading the fleet from the Mon Calamari Star Cruisers Remember Alderaan and Justice, the duo triumphed over the warlord's forces at Togoria and advanced to Lantillies on the Perlemian Trade Route, where they linked up with forces from Contruum. Having secured the Perlemian, the two admirals then split up; Kalback went to Corsin, while Burke pushed towards the Core, encountering Admiral Ledre Okins's forces in the Chazwa system. After a prolonged series of feints and raids, Burke's cunning led Okins to dispatch critical ships to the Pindra system, where they were caught in a grav trap concealed within the system's erratic asteroids. Burke succeeded in eliminating Okins at his base of operations located in the Colla system.
In 6 ABY, Burke, along with General Brenn Tantor, seized control of Reytha, Gyndine, and Commenor, positioning himself for an advance into the Core, alongside Admiral Firmus Nantz of the First Fleet and Admiral Chel Dorat of the Fourth Fleet.