Imperial Civil War
The Imperial civil war began in 2608 when the Imperial Senate split from the Emperor in a disagreement over independence being granted to a number of star systems within the Empire.
Formally independent, the star systems in question had been swallowed up during the early expansion of the Empire, either as a result of politics, or by force. Not long after he was crowned, Emperor Lorenzo III began to give option to a handful of system to detach themselves from the Empire and form their own rule.
This went against the advice of the Senate who believed that the new Emperor was undermining the power of the Imperium, and would make it appear weak in face of its "enemies". The Senate split from the Emperor and, with the aid of its supporters, launched a crusade against his rule, scourging the worlds he had set free and attempting to assassinate him.
The Senate, however, were outnumbered and three years in to their war soon began to fall to the Emperor's forces, who were led by Jason Zackaria. A cease fire was declared on both sides and talks were held.
Then, nearly a year later, the Senate launched a new offensive, one which tipped the scales heavily in their favour. In the five years that followed cities, planets and entire star systems fell to the onslaught of black-suited soldiers, until, in late 2616, Kethlan fell.