The Stellar Imperium rose out of the Terran Alliance, an old world government on Terra that began initial colonization efforts, including the Purity and other STL (slower than light) generation ships. The Alliance developed the hyper-light drive within twenty years after the initial STL launches, and began efforts to recontact the generation ships. This resulted in contact with a few human colonies, as well as the Aron. As the frontier expanded, pressure on the Alliance became more acute. There was, as yet, no reliable hyper-light communication system, so ships had to carry messages back and forth between Terra and the colonies. The Aron, which had rapidly progressed through the ranks of the Alliance military and civil service, began arguing for a more "decisive" governmental system. The Alliance government balked, but the Aron planets began revolting, as did several of the human colonies. In response, the Alliance collapsed and the Planetary Governor of Terra, Raul Diazarcan, was proclaimed Emperor. Realizing that he could not rule the expanding state from Terra, he appointed two other planetary governors as Sector Governors, giving them administrative control over regions of ten planets each, keeping fifteen under his direct control. He also provided them with then-experimental artificially intelligent megacomputers, which had been programmed to be loyal to the Imperium. Using his own three AIs, known as the Three Wise Men, he consolidated the Imperium's ruling functions within his control quickly, including assuming control of the military directly. As the new Imperium began to expand again, it encountered and absorbed the Animo, Drugan, and Legato. The military was renamed the Stellar Guard during this expansion and it began to assumed the current organizational pattern. The last race encountered was the Xenon, a warrior race whose natural abilities and technological advancement gave the Imperium it's most severe test, but not even the Xenon could overcome the tactics and equipment of the Stellar Guard. After a bloody and destructive war, the Xenon have been absorbed into the Stellar Imperium.
The year is IY 250 (IY 1 was the founding of the Imperium). The Stellar Imperium stretches over 320 systems, divided into 30 sectors, covering most of the Terran Spiral Arm. The Xenon have been conquered for 20 standard years, and have become valued members of the Imperium. The Stellar Guard has swelled to levels not seen since the Xenon War. There are no major races that remain outside the Imperium. Trade and commerce flourish. However, rebellions have become increasingly common on the Rimward frontier, and the Stellar Guard has committed large bodies of troops to the region. Also, there are rumors of something else, something just outside the surveyed borders of the Imperium.