The Rohirrim, why horses?

[Goldberry123]

Originally Posted by Morthoron

The Rohirrim have horses because the Anglo-Saxons lost to the Normans. Tolkien felt that if King Harold and his Housecarls had had a standing cavalry in 1066, they would not have lost the Battle of Hastings, and England, to the invading William the Bastard and his motley band of Norman barons and continental freebooters. Thus, the Anglo-Saxon monarchy would have retained sovereignity over England, and remained to subjugate, overtax and make lives miserable for the peasantry, rather than have foreigners do the same except much more efficiently.

Thank you for that theory. Do you have a title of a book or article in which Tolkien expressed this opinion, so I could read up in full?