Bio-hazard

The Earth once supported life on its surface. It was once a habitable planet for humans, once. Mars, the planet the inhabitants of Earth planned to Terraform. Thier world was plagued with war and toxic chemicals. The governments fled to Mars once it was finished and completed. Only the wealthy of civilians could afford the leave the Dying planet. Those that left never returned to help those who couldn't.
The soldiers never left the battlefields, the civilians were left for dead. Earth was now dying. Mars is now teaming with life and humans. Earth was never the same again.
Until now ...