[The Squatter of Amon Rûdh]

You're spot on about the Easterlings: they're mostly infantry with some cavalry and elephants. However, my reference was to Tolkien's statement in the letter quoted by Mr. Underhill:

In any case if you want to write a tale of this sort you must consult your roots, and a man of the North-west of the Old World will set his heart and the action of his tale in an imaginary world of that air, and that situation : with the Shoreless Sea of his innumerable ancestors to the West, and the endless lands (out of which enemies mostly come) to the East.