Absalom had put Amasa in charge of the army to replace Joab. Amasa was the son of a man named Ithra, the Ishmaelite who lived with Abigail, the daughter of Nahash and sister of Zeruiah, Joab's mother.
When David arrived at Mahanaim, he was welcomed by Shobi, son of Nahash, from Rabbah of the Ammonites, Machir, son of Ammiel, from Lo-debar, and Barzillai the Gileadite from Rogelim.
This was the allocation to the tribe of Manasseh, Joseph's firstborn son. Machir was Manasseh's firstborn son who was the father of Gilead. Because Machir had been an excellent fighter, Gilead and Bashan had already been allocated to him.