The regions of the east that helped out Sauron were Numenorean colonies.

Umbar was, Khand was not but it was iirc ruled by nazgul, Easterlings and Haradrims actually had a very good reasons to ally with Sauron because every time Gondor had a good period, it invaded them. Also depictions of Haradrim iconography suggest it was just one faction out of that huge land.