Jamieson, Fausset and Brown on Joshua 13:8

8. With whom--Hebrew, "him." The antecedent is evidently to Manasseh, not, however, the half-tribe just mentioned, but the other half; for the historian, led, as it were, by the sound of the word, breaks off to describe the possessions beyond Jordan already assigned to Reuben, Gad, and the half of Manasseh (see on Nu 32:1; Nu 32:33; also see De 3:8-17). It may be proper to remark that it was wise to put these boundaries on record. In case of any misunderstanding or dispute arising about the exact limits of each district or property, an appeal could always be made to this authoritative document, and a full knowledge as well as grateful sense obtained of what they had received from God (Psa 16:5, 6).