Now Absalom had set Amasa over the army instead of Joab. Amasa was the son of a man named Ithra the Ishmaelite, who had married Abigal the daughter of Nahash, sister of Zeruiah, Joab’s mother.
Here is, I. The transporting of David and his forces over Jordan, pursuant to the advice he had received from his friends at Jerusalem, Sa2 17:22. He, and all that were with him, went over in the night, whether in ferryboats, which probably always plied there, or through the fords, does not appear.
Commenting on 2 Samuel 17:22-29
So Israel and Absalom pitched in the land of Gilead. Which belonged to the tribes of Reuben and Gad, and the half tribe of Manasseh; here the Israelites that were with Absalom encamped, in order to give battle to David and his men. So Israel and Absalom pitched in the land of Gilead.
Amasa--By the genealogy it appears that this captain stood in the same relation to David as Joab, both being his nephews. Of course, Amasa was Absalom's cousin, and though himself an Israelite, his father was an Ishmaelite (Ch1 2:17). Nahash--is thought by some to be another name of Jesse, or according to others, the name of Jesse's wife.