In the LOTR world, those who fought for Sauron were not only orcs and similar creatures (like trolls). There were also two human nations, the Easterlings (inhabitants of Rhûn, east of Mordor) and the Haradrim (inhabitants of Harad, south of Mordor).
I wonder, why did they fight for Sauron? I've encountered several claims/guesses/explanations:
1. They were enslaved by Sauron.
2. They had an old conflict with Gondor, and Sauron promised them the land of Gondor in exchange for them fighting for him.
3. One of the Nazgûl was an Easterling (Khamûl the Easterling) and when he became a servant of Sauron, his people followed suit. This would not explain why the Haradrim fought for Sauron though.
Does anyone here know the answer to this? Did Tolkien ever give an answer?