The Great Galactic War dragged on for decades. Thousands of Jedi and Sith were slain. Countless star systems were ravaged.
—Jedi Master Gnost-Dural
The First Great Galactic War was a conflict between Galactic Republic and Sith Empire. The war was orchestrated by Sith Emperor Raksha Sectre centuries ago to take vengeance on the Republic and Jedi Order. The war lasted for twenty-eight years, starting in 3,681 BBY and ending in 3,653 BBY with signing of Treaty of Coruscant.
You must go where Revan did, into the Unknown Regions, where the Sith, the true Sith, wait in the dark for the great war that comes.
—Kreia, to the Jedi Exile
With the defeat of the Ancient Sith Empire during Great Hyperspace War, only one Dark Lord survived, Raksha Sectre, who promised a vengeance to the Republic and the Jedi Order. Prolonging his life by the dark side of the Force, Sectre was able to rule for 1,300 years, forming his own Sith Empire in the Unknown Regions to create a immense Sith armada, able to swiftly defeat the Republic and gaining control over the galaxy. Raksha Sectre, proclaiming himself as Sith Emperor, tried to weaken the Republic by unleashing behind the curtains Mandalorian Wars, Jedi Civil War and First Jedi Purge, but all the conflicts were stopped by Jedi Knight Revan and female Jedi General known as Jedi Exile.
The plans were foiled, but the vengeance was not, and Sith Emperor simply continued preparing his forces for next three hundred years while sending many spies to the known space to undermine the Republic and Jedi Order, aswell as destroying a spirits of previous Dark Lords such as Naga Sadow, preventively attacking them to guarantee Sectre will dont have to fight another Dark Lord of the Sith. In 3,681 BBY the preparations for the Great War were over and Sith made a first move against their eternal enemies, Jedi Knights and the Republic protected by the Jedi.
Beginning - Onslaught of the Sith Empire
In 3,681 BBY, the Sith fleet slowly advanced towards the Outer Rim, Tingel Arm. Republic noticed a presence of unknown warships and send a diplomatic convoy, but the Sith destroyed them after diplomats send back the images of the mighty Sith armada. Immediately after this act, the Sith began an assault on the Aparo sector, defeating the all Republic allies before even the Republic Navy would respond. Still, the Republic send Tingel Region Fleet to regain their lost planets and defeat the attackers.
The authorities of Sernipidal, Belkadan and Ruuria were servants of the Sith Empire and along with Sith fleet trapped the Tingel Region Fleet in the area. Realizing that they fell into the trap, Tingel Region Fleet fought desperately for survival on all sides. The Imperial Navy arrived to help their allies and engaged the Republic starships and even Republic send reinforcements from entire region, the Republic Navy was forced to retreat. The war has begun.
This Sith attack, which was called Tingel Arm campaign, was used as distraction to another attack, highly important for the Sith Order. Sending out all available ships into the Tingel Arm, many planets were weakened, but only one was a target of the Sith Order; Korriban, cradle of the Sith. Handful of Vanguard-class Jedi corvettes was not able to hold off the fleet of the Sith Fury-class Sith transports. All Jedi Knights guarding the ancient planet were slain and their ships destroyed, and the Sith after three hundred years stood on the surface of their homeworld. Soon, they rebuilt a new Sith Academy, where the training of the new Sith took place. To be even worst, the Fall of Korriban was so quick that the fact Jedi lost control over Korriban without the Jedi Council even knowing about it.
When Jedi Order learned about the loss of Korriban, the Republic finally knew who this new aggressor truly is; the Sith Empire. The Jedi Knights finally believed the legend said by Revan and Jedi Exile centuries ago of the "True Sith" hiding in the shadows of Unknown Regions was true, and rallied entire Order to prepare for inevitable confrontation with their eternal nemesis.
With most of the Republic forces in full retreat, Sectre made another step in his master plan to gain control over the Outer Rim, and send his fleets into the Sluis Van to destroy local Republic shipyards, into the Minos Cluster to seizure of rare resources here and on the Rimma Trade Route to cut off the supply lines into the Core, and more attacks followed. In all cases, Republic forces were paralyzed by Sith's swiftness and were unable to react and instead fell back to regroup.
The Jedi Council and Senate, seeing what is going on in the Outer Rim, quickly organized an emergency session to discuss about the defense of the Republic. Unfortunately, the meeting quickly took a negative turn, with the representatives refusing to participate if their worlds will not gain a priority defense. Many senators, including Itohrians, have left the meeting, electing not to contribute their military forces to the war effort against the Sith Empire. This was perhaps the greatest genius of the Sectre's plot. He had turned the Republic against itself.
Mid Rim Campaigns
Sith Empire started attacking Republic systems in the Mid Rim, and Republic was not able to effectively hold them off. The Mid Rim was systematically took over by Sith, and only on few planets Republic was able to fight with Sith with even chances, such as during Hoth Campaign. Several clashes above this remote planet between Republic and Imperial fleets occurred, and many most advanced, most powerful and experimental starships was destroyed there, on both sides. But in the end, Sith won. Than some of the battleground included Ilum, Dathomir and also Manaan, despite the Selkath wanting to remain neutral in the conflict, but Sith feared Selkath's kolto would be freely shipped to the Republic. Republic planets of Utapau and Agamar were defeated and continuously harassed.