Absalom had appointed Amasa over the army in place of Joab. Amasa was the son of a man named Ithra, the Ishmaelite who had married Abigail, the daughter of Nahash and sister of Zeruiah the mother of Joab.

2 Samuel 17:26
So the Israelites and Absalom camped in the land of Gilead.

2 Samuel 19:13
And say to Amasa, 'Aren't you my flesh and blood? May God punish me, and ever so severely, if from this time you are not the commander of my army in place of Joab!'"

2 Samuel 20:4
Then the king said to Amasa, "Summon the men of Judah to come to me within three days, and be here yourself."

2 Samuel 20:9
"Are you well, my brother?" Joab asked Amasa. And with his right hand Joab grabbed Amasa by the beard to kiss him.

1 Kings 2:5
Moreover, you know what Joab son of Zeruiah did to me--what he did to Abner son of Ner and Amasa son of Jether, the two commanders of the armies of Israel. He killed them in peacetime to avenge the blood of war. He stained with the blood of war the belt around his waist and the sandals on his feet.

1 Kings 2:28
When the news reached Joab, who had conspired with Adonijah but not with Absalom, he fled to the tent of the LORD and took hold of the horns of the altar.

1 Kings 2:32
The LORD will bring his bloodshed back upon his own head, for without the knowledge of my father David he struck down two men more righteous and better than he when he put to the sword Abner son of Ner, commander of Israel's army, and Amasa son of Jether, commander of Judah's army.

1 Chronicles 2:16
Their sisters were Zeruiah and Abigail. And the three sons of Zeruiah were Abishai, Joab, and Asahel.

And Absalom made Amasa captain of the host instead of Joab: which Amasa was a man's son, whose name was Ithra an Israelite, that went in to Abigail the daughter of Nahash, sister to Zeruiah Joab's mother.