Rebellion
The Rebellion, also known as the Second Civil War (to distinguish it from the First Civil War) and the Final War, was the undoing of the Imperium.
The moment that the rebellion started can be determined quite specifically: video cameras recorded the event, and precise timers noted exactly when the first shot was fired. At 1517 hours on 132-1116, Dulinor, the Archduke of Ilelish, fired four shots from his personal sidearm, and in succession, assassinated the Emperor Strephon, the Empress lolanthe, the Grand Princess Ciencia Iphegenia, and the Aslan Yerlyaruiwo ambassador. The throne room was in immediate turmoil.
Most of 1116 was taken up in the transmission of the news from Capital to all parts of the Imperium. As the news was absorbed, the many constituent factions of the Imperium made choices for their next course of action.
In 1117 the basic factions established themselves.
In 1118 each faction struggled to strengthen its own position while weakening all the others.
In 1119 the Rebellion settled down to a dedicated battle to the death between Lucan and Dulinor. Other factions contributed their forces to resolving squabbles between them, picking away at Lucan or Dulinor or both, or just trying to survive the chaos that was enveloping the empire.
By 1120 the factions had destroyed the mighty Imperial Navy. At the beginning of the war there were 320 numbered fleets and an equal number of reserve fleets, by the end of the year there were fewer than 95 numbered and 130 reserve fleets, and those reduced to 60% strength or less.
In 1121 the factions continued to hammer away at each other with what force they had left. However, limited resources dictated warfare to devolve into banditry, surgical strikes and terrorism.
1122 saw the start of the Black War, the lines separating war, terrorism, and piracy &mdash always thin to begin with &mdash began to vanish. Economic conditions, already shaky from years of warfare, worsened.
The Long-Term Rebellion
Each faction of the Rebellion quickly established a base of power. At and within a reasonable radius of that established base of power, the faction could expect to hold a fairly solid base of power. That reasonable radius, computed based primarily on travel times, works out to about 20 parsecs for each faction.
Beyond 20 parsecs from faction's base of power, controlling and directing worlds becomes quite a difficult proposition for the faction. Fleets need orders; armies need supplies and support; and, in addition, attacks need coordination. And the farther the battlefields are moved from the base of power of the faction, the more difficult they are to direct and control. Losing battles is expensive, but on the other hand, so is winning them. Even committing fleets and forces uses up resources that might be needed by the faction more desperately later. The least expensive course of action is to cautiously prepare, building strength while waiting for the other side to make a mistake.
Beyond the roughly 20 parsec radius of power, the territory of the Imperium became a no man's land, left alone, completely to its own devices.
End of the War
The war's final chapter is the release of the Virus as part of a raid on a weapons development facility in 1130. The resulting Collapse destroyed what remained of the Third Imperium.