Great Galactic War

The Great Galactic War, more commonly known as the Great War, was a war between the Sith Empire and the Galactic Republic between 3,681 BBY and 3,653 BBY, ultimately lasting for a total of 28 years.

Canon Information on the Great Galactic War
Wookieepedia Article See also the articles linked in this on regarding the history for game history background.

History
With the Sith Empire's defeat in the Great Hyperspace War, the Sith were on the verge of collapse. But one Sith Lord escaped the Republic and made it to the Unknown Regions. There, he re-founded the Empire, himself assuming the title of Sith Emperor, and created a council of other Sith Lords. Using the power of the dark side, the Emperor kept himself alive for over a thousand years after the Great Hyperspace War, rebuilding the Empire's strength and waiting for a chance to avenge the Sith's defeat.

The Republic, meanwhile, was challenged in a number of wars over the next few thousand years. After the war with Exar Kun, the growing Sith Empire goaded the Mandalorians to attack the Republic. After a group of Jedi defeated the Mandalorians, the Sith Emperor met with and corrupted their leaders and sent them back to act as the vanguard for his invasion, resulting in the Jedi Civil War. The Republic was also burdened by the upheaval of the Kanz Disorders.

In the year 3,681 BBY, the Sith Emperor saw that the chance for revenge against the Republic and the Jedi had come, and launched an invasion of the Republic.

SW: The Great Galactic War RPG(Non-canon)
The True Sith's invasion caught the Republic by surprise. When Sith battlecruisers first appeared in the Tingel Arm, the Republic dispatched a lightly escorted diplomatic team to see what was going on. The Sith Emperor, in an attempt to scare the Republic, allowed the diplomats to transmit images of his forces back to their superiors before destroying them.

(MORE TO BE ADDED AS THE GAME IS PLAYED)