His kingdom: This is the region around the Tigris and Euphrates rivers in southern Mesopotamia. This kingdom eventually expanded into northern Mesopotamia (Assyria).
Babylon is the most important city because it played a key role in the story of the tower of Babel (see 11:4 and study note).
Erech, known as ancient Uruk, is now Warka in southern Iraq (see Ezra 4:9–10).
Accad, once called Agade, is north of Babylon and was home to the famous ruler Sargon (2370–2295 BC).
The location of Calneh is uncertain. But it is probably one of Nimrod’s cities north of Aram-naharaim in southern Mesopotamia (compare Amos 6:2).
built … Nineveh: Like Cain, Nimrod built cities (see Genesis 4:17 and comments). Nineveh was an ancient Assyrian city on the east bank of the Tigris River in northern Iraq.
Rehoboth-Ir was either a nearby city or a daughter-city of Nineveh.
Calah is modern Tell Nimrud, south of Nineveh.
Resen is possibly modern Selamiyeh, northwest of Tell Nimrud.