What happens when you can't conquer regions?
You always have the option of picking up all your pieces and starting again from the edge of the board. You would probably be able to fairly quickly conquer all regions controlled by your opponent's inactive race.

From the rules, Page 6 - "Abandoning A Region" 


  If the player chooses to abandon all the Regions he previously
  occupied, his next conquest must follow the same rules as its First
  Conquest