Chickenpox wipes out the Martian civilisation
Across the first expeditions the Martians kill or trap the Earthmen: one jealous husband shoots the first captain, and the third expedition is lured into illusions of their dead families and murdered. By the fourth expedition in 2001 nearly every Martian is already dead of chickenpox carried by the earlier crews, so the colonisation that follows meets almost no one. The deciding event is not a battle. An ordinary childhood disease empties the planet before any policy is made.
