Joab in Wikipedia

Joab (Hebrew: יוֹאָב, Modern Yo'av Tiberian Yôʼāḇ ; "Yahweh is father") was the nephew of King David, the son of Zeruiah in the Bible. He was made the captain of David's army (2 Samuel 8:16; 20:23; 1 Chronicles 11:6; 18:15; 27:34). He had two brothers, Abishai and Asahel. Asahel was killed by Abner, for which Joab took revenge by murdering Abner (2 Samuel 2:13-32; 3:27). However, according to Josephus, in Antiquities, Book 7, Chapter 1, Joab had forgiven Abner for the death of his brother, Asahel, the reason being that Abner had slain Asahel honorably in combat after he had first warned Asahel and had no other choice but to kill him out of self defense. This battle was part of a civil war between David and Ishbosheth, the son of Saul. After this battle Abner switched to the side of David and granted him control over the tribe of Benjamin. This act put Abner in David's favor. The real reason that Joab killed Abner was that he became a threat to his rank of general. He then justifies it later by mentioning his brother. After leading the assault on the fortress of Mount Zion, he was promoted to the rank of General (1 Chronicles 27:34). He led the army against Syria, Ammon, Moab and Edom. He also took part in David's murder of Uriah (2 Samuel 11:14-25)...

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