Charles Ellicott Commentary Jeremiah 51:28

"Prepare against her the nations, the kings of the Medes, the governors thereof, and all the deputies thereof, and all the land of their dominion." — Jeremiah 51:28 (ASV)

All the land of his dominion. —The use of the singular pronoun indicates that the prophet recognizes the fact that the kings, captains, and rulers (see Note on Jeremiah 51:23) are all under one sovereign leader— i.e., under the king of the Medes and Persians.