From the official site of Star Wars: The Old Republic:

"A new holorecord in the Jedi Archive tells the story of the Sith Empire’s surprise attack at the start of the Great War over 28 years before the Sacking of Coruscant. Master Gnost-Dural notes the brilliance of the Imperial military’s carefully orchestrated assault which put the Republic at a disadvantage for the duration of the conflict.

BTC 28 – Centuries after being driven into exile, the Sith Empire returned from deep space to launch the most calculated military assault in Republic history. Strategically revealing its attack in a menacing manner, the Sith struck fear into the hearts of the Republic’s defenders even while orchestrating surprise attacks at carefully targeted locations across the galaxy. When the Republic fleet rushed to respond, they discovered the Sith had established puppet governments on several planets in the Outer Rim. Surrounded by enemies and traitors, the Republic fleet was forced to scatter, allowing the Sith Empire to sweep through most of the Outer Rim without further challenge. The Empire’s initial victories put the Republic at a disadvantage for the duration of the Great War.

Watch the sixth Timeline holorecord, Onslaught of the Sith Empire, narrated by actor Lance Henriksen."