So here's a quick introduction to the Galaxy of Fire and the Third Imperium of Man (pro-Imperial historians writing during the golden days of the Pax Humana count the Roman Empire as the first Imperium and the Holy Roman Empire as the second).
Alright you Space Dogs, listen up! Here's how it was...
Considered the 100 year's madness. A time of general chaos and disorder, the movements of peoples and the burning of cities. Out of the chaos, and the ashes of the old order, arises the Alliance. The Alliance brings law, order, and prosperity with only one demand; all fission and fusion weapons belong to the Alliance. Scientists discover what they think is a crashed space-craft at the bottom of the Pacific ocean. The news media runs with the story but it is eventually forgotten.
The Alliance become a true one world government and is now called the Earth Alliance. Fusion power, anti-gravity, nano-scale machines, and other advanced technologies become practical for the first time. The Earth Alliance begins exploring, exploiting, and settling the Solar System on a mass scale (the money is considered stimulative spending to energize destroyed economies and and the effort a collective project to unite mankind). Obviously extra-terrestrial ruins are found on Mars, Ganymede. Arkham Institute scientists rework Heim Theory to include poly-dimensional non-euclidean mathematics and test a working faster than light propulsion system.
An artifact is found on Pluto that baffles scientists for decades. Eventually an archaeologist and expert on extinct languages determines the writing on the artifact to be related to certain languages of the Ponape peoples of Polynesia. Eventually scientists manage to activate the jump gate opening up the wider galaxy. Combined with Heimdall Space-Time Warp drive the Jump Gates leads to a rush to explore the 'Final Frontier'.
First Contact War: Initial encounters with the Khai Dominion turn violent, leading to a multi-decade vicious war for survival. Just when it became obvious the alien foe had learned of Terra's location and were advancing towards it, shocking news ends the war. The Khai are members of a Galactic Council which stops the war (mostly out of factional politics, lest the Khai faction wax too powerful). Negotiations are long and tortuous but the errors of both sides are revealed and public amends are made. The Earth Alliance joins the Galactic Council as an associate species under the patronage of the Elder-Yith faction. Mankind spreads throughout the Orion Spur of the Perseus Arm of the Milky Way Galaxy. Friends are made, like the Ae-Shi and the Gnophkek, and enemies like the Ayayig, Migoul Pirates, and the aquatic Dagon. Human Psions appear for the first time.
Mankind is a rising power in the Galaxy, provoking awe and jealousy in equal measure. It becomes obvious to all that Humans are on a fast track to full Council Membership, with a seat and a vote, in a matter of a few brief centuries, unheard of. Events in the mid-Century cast doubts on this inevitability however. One man is at the center of everything; Captain Phillip Magnus Sherridan-Windsor, Commander of Byzantium Nine, a critical Star-Fort at the edge of Human Space and a trade hub in orbit around Epsilon Erridani. Phillip and his allies would both expose and thwart the design of a rogue Khai Inquisitor (agents of the Council, autonomous and deniable) to awaken slumbering terror. At the same time as Sherridan is saving the Galaxy and becoming the first Human Inquisitor, the Earth Alliance is falling under a shadow. A power mad vice president arranges for his superior to be assassinated and assumes dictatorial powers; beginning a reign of bloody terror. President-for-life Clark is only brought down when Admiral Sherridan overthrows him in a lighting fast civil war and counter-coup. The Alliance Navy hails Sherridan as Imperator and he crowns himself, thus inaugurating the Imperium.
The Human Imperium waxes strong and virile under the guidance of the Sherridan-Windsor dynasty. Now with a full council seat Mankind is truly a power astride the stars. New worlds are settled, grow, and found colonies of their own. The center of Imperial power shifts to Terra Nova, whith Old Terra remaining as a cultural and economic powerhouse. This veritable golden age of growth, discovery, advancement and achievement is soon referred to as the Pax Humana. Humanity begin to upset the old balances of power and ancient assumptions. Although still not as technological advanced as the Elders, Man is advancing at a rapid pace and beginning to question assumption older than the species itself. Many of the younger races flock to Humanity's banner, basking in the reflected glory that a youngling race can do so much.
Shadows grow long and wheels turn within wheels, the golden age turns ashen as suspicions grow amongst the stars. Something is amiss in the black; ships disappear and colonies go silent. Trade is disrupted and voices and hands are raised against each other;
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.
And what rough beast, its hour come round at last,
Slouches towards Gomorrah to be born?