As humanity progressed we found that earth was a wasteland and we failed to fix it. Climate change was starting to become more evident and sea level started to rise. However, as this happened Hyperspace travel was discovered and the rich, powerful and the governments of earth began building colony ships, cities were abandoned and much of humanity left the planet to settle new worlds that were not as damaged and could be terraformed into new homes. The thought of new planets with no competition was also a major factor in the exodus for many ideologies. Shipbuilding companies worked hard for many years and soon only about 46% of the population of earth remained on the planet, most of them unable to escape.
It seemed inevitable that they would watch nature die, and they thought that they would soon follow. However, when humanity left they didn't take their militaries with them due to the fact that those militaries were not needed, trusted and had been used to keep colony ship launches safe. So with large numbers of military personnel left behind several military dictatorships were created, and then destroyed by the combined might of the western militaries that had bonded together under the name of the United Earth Directorate. This new government took the population of earth and rallied them together into a utilitarian society that began to rebuild. With so many people gone most of humanity were moved to the industrial cities and many nature habitats began to recover. Working hard and under strict rule the UED put humanity back in place and resumed the shipyards and rebuilt the damaged cities and infrastructure. Soon the first Human warship was constructed and tasked with bringing a small number of troops to Alpha Centauri to capture any human equipment there. This ship captured a large station that had been built with a large bio dome and a automated mining station. This station was a small colony that became the first conquered state of the UED. Unfortunately, the station had long ago lost contact with the human colony ships. On earth with humanity's might The UED rebuilt the population, increased thier technology and thier fleet grew. The ecosystem was recovering but still badly damaged. In this time a small Xenoid hive ship landed on the earth and a small war was fought with the xenoids being killed.
Almost 80 years after the colony ships left Earth the UED launched the first Invasion fleet to bring every human colony back into the rule of earth and so began the first and most important mission of the UED, create and maintain and unified human government to keep humanity unified and strong.
Over the next 100 years UED Invasion fleets would go out to the often under-armed colonies of humanity and bring them into the fold.
These days earth is still a mostly dead world with massive cities that require artificial methods to replace trees and nature. But nature slowly returns, different as it took so much damage and was so polluted but coming back. The UED maintains a large empire and military.
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The United Earth Directorate
Science FictionA faction of my science fiction setting