A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown [1882] at sacred-texts.com
Now the sons of Reuben--In proceeding to give this genealogy, the sacred historian states, in a parenthesis (Ch1 5:1-2), the reason why it was not placed first, as Reuben was the oldest son of Jacob. The birthright, which by a foul crime he had forfeited, implied not only dominion, but a double portion (Deu 21:17); and both of these were transferred to Joseph, whose two sons having been adopted as the children of Jacob (Gen 48:5), received each an allotted portion, as forming two distinct tribes in Israel. Joseph then was entitled to the precedency; and yet, as his posterity was not mentioned first, the sacred historian judged it necessary to explain that "the genealogy was not to be reckoned after the birthright," but with a reference to a superior honor and privilege that had been conferred on Judah--not the man, but the tribe, whereby it was invested with the pre-eminence over all the other tribes, and out of it was to spring David with his royal lineage, and especially the great Messiah (Heb 7:14). These were the two reasons why, in the order of enumeration, the genealogy of Judah is introduced before that of Reuben.
Eastward he inhabited unto the entering in of the wilderness from the river Euphrates--The settlement was on the east of Jordan, and the history of this tribe, which never took any part in the public affairs or movements of the nation, is comprised in "the multiplication of their cattle in the land of Gilead," in their wars with the Bedouin sons of Hagar, and in the simple labors of pastoral life. They had the right of pasture over an extensive mountain range--the great wilderness of Kedemoth (Deu 2:26) and the Euphrates being a security against their enemies.
THE LINE OF GAD. (1Ch. 5:11-26)
the children of Gad dwelt over against them--The genealogy of the Gadites and the half-tribe of Manasseh (Ch1 5:24) is given along with that of the Reubenites, as these three were associated in a separate colony.
Sharon--The term "Sharon" was applied as descriptive of any place of extraordinary beauty and productiveness. There were three places in Palestine so called. This Sharon lay east of the Jordan.
upon their borders--that is, of Gilead and Bashan: Gilead proper, or at least the largest part, belonged to the Reubenites; and Bashan, the greatest portion of it, belonged to the Manassites. The Gadites occupied an intermediate settlement on the land which lay upon their borders.
All these were reckoned . . . in the days of Jotham--His long reign and freedom from foreign wars as well as intestine troubles were favorable for taking a census of the people.
and in the days of Jeroboam--the second of that name.
Hagarites--or, "Hagarenes," originally synonymous with "Ishmaelites," but afterwards applied to a particular tribe of the Arabs (compare Psa 83:6).
Jetur--His descendants were called Itureans, and the country Auranitis, from Hauran, its chief city. These, who were skilled in archery, were invaded in the time of Joshua by a confederate army of the tribes of Reuben, Gad, and half Manasseh, who, probably incensed by the frequent raids of those marauding neighbors, took reprisals in men and cattle, dispossessed almost all of the original inhabitants, and colonized the district themselves. Divine Providence favoured, in a remarkable manner, the Hebrew army in this just war.
the God of Israel stirred up the spirit of Pul--the Phalluka of the Ninevite monuments (see on Kg2 15:19).
and the spirit of Tilgath-pilneser--the son of the former. By them the trans-jordanic tribes, including the other half of Manasseh, settled in Galilee, were removed to Upper Media. This was the first captivity (Kg2 15:29).