J. R. R. Tolkien made use of classical era, medieval and recent sources, including his own experience, in creating the places and peoples of Middle-earth. The main influences identified by scholars are shown on the map. Many Middle-earth places derive from several sources. 
Main reference: Garth, John (2020)       The Worlds of J.R.R. Tolkien: The Places that Inspired Middle-earth, Frances Lincoln Publishers &amp; Princeton University Press, pp. 12–13 The Shire, 39 Harad, 41, 151 Gondor, 32 Dwarves, 30 Trolls, 37 Numenor, 55 Elves, 88 Misty Mts, 159–168, 175, 182 Mordor, 32, 126 Rohan  ISBN:  978-0-7112-4127-5. 
Other references: Librán-Moreno, Miryam (2011) "'Byzantium, New Rome!' Goths, Langobards and Byzantium in The Lord of the Rings" in Fisher,  Jason , ed.    Tolkien and the Study of his Sources, MacFarland &amp; Co., pp. 84–116  ISBN:  978-0-7864-6482-1.  Gondor as Byzantium, Goths
Letters of J.R.R. Tolkien #168 Gondor as Venice
Bowers, John M. (2019)       Tolkien's Lost Chaucer, Oxford University Press, p. 170  ISBN:  978-0-19-258029-0.  Harad as Barbary Corsairs
Salo, David (2004). "Heroism and Alienation through Language in The Lord of the Rings". In Driver, Martha W.; Ray, Sid (eds.). The Medieval Hero on Screen: Representations from Beowulf to Buffy. McFarland. pp. 23–37. ISBN 978-0-7864-1926-5. Gondor as Byzantium; Umbar (of Harad) as Carthage; Southrons (Harad) as Arabs; Easterlings as Sarmatians, Huns, and Avars.
Solopova, Elizabeth (2009), Languages, Myths and History: An Introduction to the Linguistic and Literary Background of J. R. R. Tolkien's Fiction, New York City: North Landing Books, ISBN 0-9816607-1-1, pp. 70–73: Pelennor as Chalons/Catalaunian Fields
Swycaffer, Jefferson (1983). "Historical Motivations for the Siege of Minas Tirith". Mythlore 10.: Mordor as Asia Minor; Rohan as Hungary; Gondor as Constantinople
Own work
Ian Alexander