The First Defense Fleet served as a fleet within the Imperial Navy of the reconstituted Sith Empire, tasked with the protection of Dromund Kaas, which functioned as the Empire's capital planet. In the year 3640 BBY, amidst the Galactic War being waged against the Galactic Republic, this fleet engaged in combat against the Republic Navy when an invasion led by Jedi Grand Master Satele Shan attacked Dromund Kaas. While the Defense Fleet proved largely effective throughout the engagement, dealing substantial damage to the opposing forces, the Republic fleet retreated upon achieving their actual objective: the defeat of the Sith Emperor.
The initial reference to the First Defense Fleet occurred within the Dromund Kaas planet summary during "Doomsday," a story mission available to the Jedi Knight class in the video game Star Wars: The Old Republic. This MMORPG was jointly launched by BioWare and LucasArts in 2011.