The Eighty-sixth Infantry served as a division within the Republic Army during both the Cold War era and the Galactic War against the Sith Empire, both conflicts involving the Republic. Back in 3642 BBY, a specific group from this division, stationed at the Adrenal Research Base on the planet Quesh, found themselves defending their location. This group was under the command of Lieutenant Gundo alongside the Jedi Sajar, and the defense was necessary due to an attack by Imperial forces as part of the Empire's invasion of Quesh.
During the Cold War between the Galactic Republic and the Sith Empire, the Eighty-sixth Infantry served as a division within the Republic Army. It was one of the many units positioned at various Republic military installations throughout the galaxy. As the year 3642 BBY arrived, marking the start of the Galactic War between these two galactic powers, a particular detachment, led by Lieutenant Gundo of the Mirialan species, was assigned the task of protecting the Adrenal Research Base located on the planet Quesh. Also stationed at the base was Sajar, a Jedi Padawan who had previously been a Sith Lord and apprentice to Jedi Master Tol Braga. His assignment was to meditate on the light side of the Force while simultaneously serving the Republic. Gundo's detachment included several engineers who collaborated with the base's personnel. When the Empire launched its invasion of the planet, aiming to escalate the conflict into full-scale war with the Republic, these soldiers were thrust into combat.
When Gundo's troops managed to capture some Imperial scouts, Sajar inadvertently gave in to the dark side and killed them during interrogation. His subsequent taunts, fueled by rage, directed at the Imperial forces only provoked the attackers further. Once he regained his composure, Sajar was filled with horror and refused to engage in combat, fearing he would further succumb to the dark side. This left Gundo and his troops facing a significantly larger enemy force. However, with the assistance of the Jedi Knight known as the Hero of Tython, dispatched by Braga to investigate his apprentice's silence, the Eighty-sixth Infantry managed to repel the Imperial attackers. They also established makeshift ray shields to safeguard the facility until reinforcements could arrive.
Being an infantry division within the Republic Army, the Eighty-sixth was equipped with standard trooper armor. The division's armor was distinguished by its white, gray, and gold coloring. Furthermore, many soldiers were armed with standard-issue blaster rifles.
The Eighty-sixth Infantry made its initial appearance in the Jedi Knight class mission titled "The Emperor's Wrath" within Star Wars: The Old Republic, a video game that was launched in 2011 by BioWare and LucasArts.