2 Samuel 19:13

Say ye to Amasa. — Amasa, like Joab, was David’s nephew, although possibly his mother may have been only half-sister to David. In this offer of the command-in-chief to the rebel general, David adopted a bold, but a rash and unjust policy. Amasa should have been punished, not rewarded for his treason. He had given no evidence of loyalty, nor was there proof that he would be trustworthy. Moreover, this appointment would be sure to provoke the jealousy and hostility of Joab. But David had long been restless under the overbearing influence of Joab (see 2 Samuel 19:22; 2 Samuel 16:10; 2 Samuel 3:39), and now since he had murdered Absalom, was determined to be rid of him. He therefore took advantage of the opportunity by this means to win over to himself what remained of the military organisation of Absalom.