Jamieson, Fausset and Brown on 2 Samuel 19:13

13. And say ye to Amasa, &c.--This also was a dextrous stroke of policy. David was fully alive to the importance, for extinguishing the rebellion, of withdrawing from that cause the only leader who could keep it alive; and he, therefore, secretly intimated his intention to raise Amasa to the command of the army in the place of Joab, whose overbearing haughtiness had become intolerable. The king justly reckoned, that from natural temper as well as gratitude for the royal pardon, he would prove a more tractable servant; and David, doubtless, intended in all sincerity to fulfil this promise. But Joab managed to retain his high position (see on 2Sa 20:4-10).