In the sixth mission (the Dust District) you are given an option to eliminate either the head of the Howlers or the head of the Overseers in order to get the combination to a lock (you can also find the combination through other means, including logically working it out). This achievement is gained for knocking out/killing the head of the Howlers (Paolo) and returning the body to the Overseers. The Overseer territory will go neutral the moment you enter carrying Paolo's body.