Albert Barnes Commentary 1 Chronicles 5

His birthright was given... In particular, the right of the firstborn to a double inheritance (Deuteronomy 21:17) was conferred on Joseph. This happened both through the expressed will of Jacob (Genesis 48:22) and in the actual partition of Canaan (Joshua 16–17).

However, while the birthright’s material privileges passed to Joseph, its other rights—those of dignity and preeminence—fell to Judah, from whom the chief ruler came. This is an allusion especially to David, though it may also extend to include the Messiah, the true “Ruler” of Israel (Micah 5:2).

The sons of Joel - The line of succession given here must be broken by one great gap or several smaller ones, since nine generations before Tiglath-pileser would carry us back no further than the reign of Rehoboam.

He inhabited - that is, Reuben. Eastward, the Reubenites inhabited the land as far as the commencement of the great Syrian Desert, which extended all the way from the river Euphrates to their borders.

The "Hagarites" or "Hagarenes" are generally regarded as descendants of Hagar and a distinct branch of the Ishmaelites (1 Chronicles 27:30–31; Psalms 83:6).

They appear to have been one of the wealthiest (1 Chronicles 5:21) and most widespread tribes of the Syrian Desert. They were found along the Euphrates in contact with the Assyrians, and also in the Hauran near Palestine, in contact with the Moabites and Israelites.

If they are identical with the Agraei of classical writers, their name may survive in the district called Hejer or Hejera in northeastern Arabia, on the borders of the Persian Gulf. A full account of the war is given in 1 Chronicles 5:18–22.

From this passage and from the subsequent account of the Manassites (1 Chronicles 5:23–24), the Gadites extended themselves to the north at the expense of their brothers, gradually occupying a considerable portion of the tract originally allotted to the “half tribe.”