Guy Gardener wrote:
The humans used Nukes.
The Aliens in return Terraformed Earth.
They probably hadn't intended on finding a technological infesting their new home civilization when they left from parts unknown.
Prof Brown in Star Trek II was pretty sure what would happen if you tried build paradise on top of a bustling metropolis.
The Votans were in STL sleeper ships for several thousand years. They knew earth could support life, maybe but that's all they knew. Once they got here they could not leave. The fleet was stranded. They stayed in orbit for a decade negotiating with earth then a human first fanatic assassinated the Voton ambassador and war broke out. The terraforming was an accident.. When the Arks started crashing the terraforming machines went beserk and are trying to terraform the earth at random with 5 different planets ecosystems.