The Solar Empire did not exist prior to the Shay-An War. Before the war, it was a weak central government that only existed as a series of mutual defense and trade pacts between the major stellar nations of humanity, called the Terran Confederacy. The vicious and unrelenting invasion of human territory by the Shay-An demonstrated the need for a non-aligned interstellar force to deal with non-human threats.
The Shay-An War began when human ships entered the territory now known to be a part of the vast Shay-An Protectorate. The Protectorate is a region of space that the Shay-An set aside for its many client races, including a host of nascent sophonts. It was not the violation of territory that provoked the Shay-An, but the creation of colonies within the Protectorate. It demonstrated an alarming lack of foresight to the Shay-An, who deemed humans too dangerous to continue unchecked. The Confederacy’s colonies and ships within the Protectorate went silent abruptly and within a month the Shay-An Armada had begun seizing worlds near the border.
The war was devastating. Although the combined arms tactics of the Confederacy’s member states easily won planetary battles, the Shay-An enjoyed vastly superior space mobility and technology. The Confederation employed slower jump drives for FTL travel, taking a week to travel, whether it be one parsec or four. The Shay-An, however, used the more advanced warp drive method, granting them faster interstellar travel times and fuel efficiency.
If it weren’t for the sessra, humans would be a footnote in galactic history. The insatiably curious and mercantile sessra provided the human military with warp drives (for a very steep price) and assisted with the creation of biots; mass-produced robotic intelligences. Armed with equalized travel times and legions of cheap, durable troops, the Confederacy forced the Shay-An into a war of attrition; something the Shay-An could not sustain despite their technical superiority.
When the war was over, the nature of the Terran Confederacy had changed considerably. Taxes had become levied against member states to finance the explosive growth of the Confederate Military and new government structures had evolved to coordinate the massive war. The ante bellum structure had closely resembled ancient Greece; independent states loosely affiliated with each other but not always peacefully co-existing or even cooperating. The post war environment is seen to be more like the Roman Empire and so it was decided to rename it the Solar Empire in imitation of that ancient society (despite technically being a republic).
There were those that did not approve of the new militant republic. The decade immediately following the Shay-An War was one of consolidation. Human polities were won over by compromise, trickery, and at the point of a sword. It was regrettably necessary in the view of the Imperial senators; they could not afford to have a divided front in the face of the crushingly superior Holy Empire of the Shay-An. Any weakness would likely be ruthlessly exploited by the Shay-An.
The worlds and polities nearest to the present Shay-An border were eager adopters of the Solar Empire, having suffered the most from the war. These subsectors eventually became known as the Sword Worlds for having the highest per capita military forces in the entire empire; in both local forces and Imperial forces. Nobody was willing to see a repeat of the Shay-An War’s first year, where whole sectors were simply overrun by the Shay-An Armada.
The present-day Solar Empire is politically stable despite the ongoing migration of its biot population to an area of space known as the Badlands.