Matthew Poole and continuators on 1 Chronicles 5:9

9 And eastward he inhabited unto the entring in of the wilderness from the river Euphrates : because their cattel were multiplied in the land of Gilead.

i. e. The Tribe of Reuben.

From Iordan and the Wilderness beyond it unto Euphrates. Or, of the Wilderness which lies towards or reacheth to the River Euphrates, namely, the great Wilderness of Kedemoth, Deut. 2. 26. which was extended far and wide towards Euphrates: for that was the Eastern Border of Reubens Possession, and not Euphrates, to which their Habitation never reached.

Which forced them to enlarge their Habitation as far as they could Eastward towards Euphrates.