You did see the thing where Group Minds are gods, right?
Alter Terrain as a inherit action fixes the desert. The others would probably decide to launch as many attacks as they possibly could against said army, in hopes of saving the other colony..
Since after about fifty years of inter-colony war, an outside threat forced them to work together to survive. By now, most of the colony-minds are close friends.
Wouldn't you try to save the guy you've been buddies with for over a hundred years?
Though I have to admit that is a very effective tactic against nearly
any race.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather