I'm in the process of writing a backstory to go with the Human Factions Mod, available in the Library. It's going to have 4 parts, and should help explain the races and scenarios I've included.
I was originally going to have present the story as a campaign but thought the interest in the main GC2 campaign didn't equal the effort needed to make a campaign, so I would create a forum post instead.
Chapter 1: The End of The World
December 19th, 2008.
The day historians would later call the end of the world.
All across the globe, in all languages, one story dominated TV channels and radio stations.
"An elite group of scientists in The People's Republic of China claim to have developed a stable functioning nuclear fusion engine. They have filed a patent and the world awaits a decision."
But it proved to be a hoax, or at least that's what a statement from the Chinese government published on the 20th claimed.

After 10 years, the hype and conspiracy theories died down but in 2027 China launched a surprise attack on Taiwan, conquering the entire island in three hours. From then on, as their campaigns took them through southeast Asia, Indonesia, Australia and the Pacific, it became apparent they had been building a fusion powered economy and a fusion powered war machine for the past 14 years.

Despite the efforts of NATO and various other international forces, the world was not prepared for the ferocity of the Chinese campaigns. In just one year, they had captured the Asia-Pacific region, Oceania, Central America and large sections of the middle east.

Only after the USA fired 12 nuclear missiles at major Chinese urban areas did the communists halt their advances. A fragile peace was brokered and the world returned to normal.
China was readmitted to the UN and allowed to keep the spoils of the war, mainly because the UN and NATO had no way of getting the Chinese government to return them to local control.

The Chinese couldn't be kept at bay forever though, and when fusion technology was stolen by an American spy, they chose their time to strike. They threw everything they had at Europe and North America.As the war continued in areas like Greece and Mexico allied resistance intensified, with some nations utilising prototype fusion weaponry. But Britain retaliated with surprisingly small amounts of conventional forces.
It eventually emerged why. Chinese intelligence stole a blueprint for a spaceship powered by a revolutionary new fusion engine - HyperDrive.
HyperDrive is an onboard system that warps space in order to make journeys faster. To invent something like that in such a short space of time, however, many have deemed impossible and all the scientists involved in the project died in their sleep the night after the Chinese infiltration of unknown causes, so nobody knew how they achieved HyperDrive.
Once HyperDrive was stolen, Britain immediately collected colonial volunteers and on the 13th of July 2024, the rocket carrying with it the whole of Parliament and nearly one billion colonists, took off in direction of Alpha Centauri, leaving Britain in the hands of the crown.
Soon, nation after nation fled the Earth against Chinese invasions: Germany, Mexico, India, France and Japan. South Korea fled from North Korea's invasion and Russia, long-standing neutral, took its chances and launched to the stars. By 2029, only China and the USA were powers on Earth.
But, following a crippling wave of riots and the collapse of the Chinese government's authority, China left its empire in anarchy and took to the stars, thus leaving the USA as the ruler of the Earth.
Chapter 2: A New Beginning coming soon.