Humans have wiped out all resources on their planet and made it into a polluted ball that only Al Gore could love. To escape this sludgepit, they then escape to and Terraform Mars and Venus. They demand independence, Earth says no, there's a big war and all three planets are destroyed. They make a multi generational ship and travel to the nearest terraformable planet and set ourselves to wipe out the hippie peace-loving aliens on the planet below.

Dear lord Humans suck!