i had the same problem, ive almost got all the badges and i still hadnt completed that one!
Go to war with a race you intend to completely remove.
Eventually if you beat them enough they offer to surrender but stay mad at you, accept the surrender.
At that point you should be able to trigger the mission.
Finish them off.
the problem is the game wants to find someone who is pissed at you to direct you too, if youre like me you make everyone around you at least yellow, and i think theres a range limit on how far out the game is willing to look for one.