All the land of his dominion. — The use of the singular pronoun indicates that the prophet recognises 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.
Related Commentaries of Jeremiah 51:28
Jeremiah 51:28
28 Prepare against her the nations with the kings of the Medes, the captains thereof, and all the rulers thereof, and all the land of his dominion.