In BFME there was a Mordor mission aiming to reunite different Haradrim tribes in order to use them against the West. I use an extended map which goes down to what this web calls Kingdom of Raj http://lindefirion.net/maps/index.html (political map 1700 for example); and it appears to be a good solution for Harad to have to conquer parts of it's realm from independent rebels/factions before it can focus on Gondor.