The Cold War signifies an intense period spanning eleven years, characterized by astropolitical tension at a galaxy-wide scale. This tension existed between the Galactic Republic and the Sith Empire, along with their respective allies, commencing after 3653 BBY. The Treaty of Coruscant marked the conclusion of the Great Galactic War, effectively acknowledging the Sith Empire's partial triumph and its subsequent acquisition of nearly two-thirds of the galaxy known at that time. While peace was achieved, the relationship between the Empire and the Republic remained strained, particularly as the Empire expanded its control over additional planets situated within the Core Worlds. In the decade that followed the war, the galaxy was consumed by pervasive fear and uncertainty. Minor conflicts and proxy wars erupted between the Empire and Republic, suggesting that the fragile truce was on the verge of collapse, which ultimately occurred in 3641 BBY.
The Cold War's eleven-year duration stands out as a period of extreme importance in galactic history, especially concerning the Sith. Due to the overwhelming power held by the Sith during this time, it is often considered the peak of their strength.
The harsh stipulations of the Treaty of Coruscant demanded an immediate end to hostilities and the withdrawal of all Republic and Jedi forces from battle zones throughout the galaxy. As the Treaty was implemented, planets previously allied with the Republic were left vulnerable and swiftly overtaken by the Empire. Other planets, fueled by resentment towards the Senate's acceptance of the treaty, chose to sever their ties with the Republic.
Upon the Jedi's return to their devastated temple on Coruscant, they faced the anger of the Senate, who blamed the Jedi Order for the war's outcome. Unable to remain on Coruscant due to the shift in public opinion against them, the Jedi relocated to Tython, their world of origin.
Despite his Empire's unprecedented strength, the Sith Emperor appeared to have lost his ambition for complete domination. He delegated control of the Empire to the Dark Council, a group of twelve Sith Lords responsible for managing the daily operations of the state. As the Sith solidified their power, the Dark Lord pursued his own enigmatic objectives.
Border skirmishes and proxy wars between the Empire and Republic erupted, and the uneasy truce between the two foes appeared to be on the verge of collapse. The conflict included small battles in the Outer Rim and other conflicts fought by proxy. One notable example was the Separatist War on Ord Mantell, a civil war pitting the corrupt planetary government loyal to the Republic against a secessionist guerrilla force.
During the Cold War, Kel Dor Jedi Master Gnost-Dural documented the events surrounding the treaty at the request of the Jedi Council on Tython.
As the Cold War neared its end, both the Republic and Empire saw the rise of four new, influential heroes. On Tython, two exceptionally gifted apprentices, a Consular and a Knight, emerged and underwent training, becoming significant figures in the war and contributing substantially to the Republic's cause.
The Consular initiated their journey by safeguarding the Tythonian Forge from destruction at the hands of Nalen Raloch, successfully acquiring three of four holocrons, and discovering the Fount of Rajivari, which housed the contents of the fourth holocron, and more. Subsequently, the Consular rescued the Jedi Order from internal downfall by mastering an ancient shielding technique when several Jedi Masters were infected by an ancient, dark plague. Upon mastering the technique, the Consular healed the afflicted masters and tracked the plaguemaster to a lone flagship. The plaguemaster, Lord Vivicar was defeated, and the spirit of Terrak Morrhage was expelled from his body. The Consular was then named the third Barsen'thor in the history of the Jedi Order.
The Knight began by assisting the Jedi Order in defeating the Flesh Raiders and two Dark Jedi, Callef and Bengel Morr, who were responsible for controlling the Flesh Raiders during the events of the Flesh Raider uprising. The Jedi Knight then followed up with victories against the Sith, pursuing enemies across the galaxy, such as Tarnis, a Sith Lord under the disguise of a doctor. When Tarnis was slain, his father, Darth Angral, bore a personal grudge against the Knight during the resulting Desolator crisis. When Angral decided to unleash a powerful superweapon on Tython in order to destroy it, the Knight was responsible for defeating Angral, destroying the Desolator and protecting Tython. Due to this, the Knight earned the title "Hero of Tython."
Concurrently, a trooper was assigned to Havoc Squad on Ord Mantell. Havoc Squad's commander, Harron Tavus, soon betrayed the Republic and defected with the rest of Havoc Squad except for the new trooper. As the only remaining member of Havoc Squad, this trooper became the new commander and was assigned to track down the traitors while the squad was rebuilt.
A smuggler also arrived on Ord Mantell, delivering a shipment of weapons. The smuggler was immediately betrayed by Skavak and was accidentally involved in the hunt for the lost treasure of Nok Drayen, the legendary pirate and thief. The smuggler eventually claimed Nok's reliquary for themselves and, with Risha Drayen, went on to become a legend in the criminal underworld. Working as a privateer for the Republic, the smuggler eventually exposed Imperial agents in the Senate and defeated the Imperial admiral known as the Voidwolf. By accomplishing that, they gained the name "Voidhound" and claimed a massive pirate fleet and criminal empire, continuing to support the Republic.
Simultaneously with the rise of the Republic's heroes, four other exceptional individuals emerged as champions of the Empire. Two of these individuals, a former slave and a Sith Acolyte, were dispatched to Korriban and mentored by Darth Zash and Darth Baras, respectively.
The slave was sent into the dark pits of the tombs on Korriban to prove themselves worthy of being Sith. All the while, they dodged several attempts on their life by their overseer, Harkun. After gaining their apprenticeship, they went on to become a powerful Sith Inquisitor and recovered the ancient relics of Tulak Hord. At the same time, they fought many battles for the Empire across the galaxy. After defeating their master, Zash, they were named a Lord of the Sith. They were given the name Lord Kallig, and established their own power base within the Sith Order and Imperial society.
Meanwhile, the other became a potent Sith Warrior. They served their master in combating his nemesis, Jedi Master Nomen Karr, and tracking down his padawan. They traveled across the Empire and saved their master's network of spies across Republic and Hutt Space. They succeeded in bringing down many Republic allies and Jedi in their mission. After defeating Nomen Kaar, they were rewarded with the title of Sith Lord and went on to serve their master against the Republic and Jedi.
The other two, the Imperial agent Cipher Nine and an unidentified bounty hunter, appeared on Nal Hutta, the former working on a mission to secure Hutt support for the Empire, and the latter to gain sponsorship in the Great Hunt. Cipher Nine gained the title of Cipher after performing successful missions for Imperial Intelligence. After they departed Dromund Kaas, they spent the Cold War tracking down and eliminating terrorists in the Empire. After defeating all the terrorist cells and their leader, they secured the Empire against further internal attacks and gained prestige. Meanwhile, the bounty hunter became engaged in a bitter contest across the galaxy. All the while, they battled the attempts by a dishonorable Mandalorian named Tarro Blood to sabotage them. After killing Tarro and the Jedi Kellian Jarro, they were named the Grand Champion of the Great Hunt and were welcomed into the Mandalorians by Mandalore himself.
The resumption of war between the Empire and Republic was heralded by a series of proxy conflicts, including the resolution of several disputes that had remained unresolved at the conclusion of the prior war between the two major powers.
Following the end of the previous war, the Empire declared victory in their long struggle for control of the industrial world Balmorra. However, this celebration proved premature. Elements within the Republic Army refused to accept defeat, choosing instead to defy the Treaty of Coruscant by remaining on Balmorra and forming a resistance movement in collaboration with the planet's inhabitants. After more than a decade of guerrilla warfare, Republic interests ultimately prevailed when a Jedi Consular galvanized the resistance and expelled the Empire from the planet.
Simultaneously, in the Outer Rim, the Empire secured a victory of its own. For several years during the Cold War, the Republic had endeavored to transform the ruined world of Taris into a symbol of hope by rebuilding the planet following its devastation by the Sith three centuries prior, during the Jedi Civil War. The Republic's efforts ultimately proved futile, as Darth Gravus spearheaded a campaign to sabotage the project by eliminating key supporting personnel. With their deaths, the project collapsed and the Republic's efforts to restore the planet came to an end. Even as they were being driven from Balmorra, the Empire was able to claim victory on Taris.
Amidst the strain on the fragile peace, Republic Navy Fleet Admiral and Jedi Master Oteg was contacted by the spirit of the long-deceased Jedi Master Meetra Surik. Having disappeared from Republic space three centuries earlier, Surik had perished in a failed attempt to assassinate the Sith Emperor alongside her old friend and leader, the legendary Jedi Master Revan. Revan, meanwhile, had been captured by the Emperor and held in a form of stasis for the intervening centuries. With the conflict between Republic and Empire escalating once more, Surik recognized that the time had come to liberate Revan from captivity.
Following an intelligence-gathering assault on the Imperial fortress at Taral V, Oteg led his fleet into The Maelstrom to attack the Imperial prison facility inside. A fierce battle—one of the first of the soon-to-be new war—ensued, with both sides taking heavy casualties. Ultimately Oteg's fleet prevailed, successfully retrieving Revan before withdrawing from the area. With his freedom, Oteg believed that the Republic had obtained the means by which to finally defeat the Empire.
Revan's respite and sojourn to Republic space would be brief. Now fully aware of the crisis facing the galaxy, he set out to utterly destroy the Empire with the aid of ancient Rakata technology, an automated mass-manufacturing space station known as the Foundry. The Foundry was set to manufacture countless extermination droids designed with bioscanners to detect Sith genetic material, and destroy any life containing it—which would result in the genocide of the vast majority of the Imperial population.
The Empire had prepared for this situation. Darth Malgus, long opposed to the peace with the Republic, revealed to his chosen strike team that the rescue of Revan had been Malgus' own design. He intended that Revan would lead the Empire to the Foundry, in order that it could be used by the Empire in the coming war against the Republic. The Imperial strike team was successful in their attack on the Foundry, destroying the few active extermination droids already built before defeating the Jedi "hero" himself. But rather than die, Revan vanished in a mysterious burst of light and energy, his fate unknown. After mopping up the remaining Republic starships, the Empire claimed victory—and with it, ownership of the Foundry.
Elsewhere in the galaxy, Jedi Master Tol Braga initiated an ambitious plan which he hoped could pre-emptively avert the coming war: to capture and subsequently redeem of the Sith Emperor to the light side of the Force.
Braga's quest enlisted the aid of a rising Jedi star who, by that point, had become known as the Hero of Tython due to their role in saving Tython from destruction at the battle of the Oppressor. The Hero traveled the galaxy for this mission, seeking out clues as to the whereabouts and technical specifications of the Emperor's highly secretive space station, conducting investigations on Balmorra, Quesh, and finally Hoth. The attack on the Emperor's person went poorly for the Jedi strike team—the entire force was captured, and most of the Jedi were effortlessly subdued by the Emperor's admirable command of the dark side of the Force.