What happened to Mordor?

[Morthoron]

Originally Posted by Tuor in Gondolin

Actually, Gondor, and later the Rohirrim, were essentially
colonial powers in Rohan, chasing out the Dunlendings-
which is why that folk helped Saruman.

I'm not certain 'colonizers' would be the best classification for Rohan, and more specifically, Gondor. Gondor seemed to me to be more of a feudal Imperial power, receiving vassalage from the Rohirrim (who were originally colonists of Gondor, so to speak). In addition, they received tribute from various client nations (like Harad), and various lords had jurisdiction over pieces and parts of the realm (Prince Imrahil of Dol Amroth, Faramir, Prince of Ithilien, etc.).

As I said, Rohan was originally a colony of Gondor, rather like when the English exported Scots Protestants to Northern Ireland, but they were autonomous. Perhaps a better analogy would be when the Roman Emperor gave the Goths an area already abandoned by the Romans to be their homeland (the Gondorions in effect gave Rohan land they could no longer control).