Humans are a destructive species. They inevitably destroy everything they touch. Everyone knew it was coming, but when Earth finally started dying action needed to be taken. A space craft, Galileo, was sent to inhabit a far off planet. E-2391, New Earth. Fifteen years later a follow up craft was sent, Galilei.
The two ships have had no contact, Galilei had no idea if the first ship made it to New Earth. Galilei were expecting a thriving planet, hopefully inhabited by their sister ship. When they arrived they didn't find what they thought.
Galileo had arrived at New Earth fifteen years prior, but it wasn't uninhabited. Another species, human-like in appearance, yet so vastly different. As humans do, they invaded. Claiming lives and slaves of the new species. Until they faught back.
What Galilei has landed in, is a full war. New World War 1.
(Entry for @sciencefiction contest)
It has been 20,500 years since we left. It was coming. So we prepared. The space cruiser departed Earth in search of a new hope, a new planet. Of course we failed. Countless worlds were surveyed, but not one could hold human life. So, now we are back. We are home.
But this is not the planet we left.