C. F. Keil and Franz Delitzsch on 2 Samuel 20:4

2Sa 20:4

David then ordered Amasa to call the men of Judah to pursue Sheba the rebel, and attack him within three days, and then to present himself to him again. This commission was intended as the commencement of the fulfilment of the promise which David had given to Amasa (2Sa 19:14). It was no doubt his intention to give him the command over the army that marched against Sheba, and after the defeat of the rebel to make him commander-in-chief. But this first step towards the fulfilment of the promise was a very imprudent act, like the promise itself, since Joab, who had been commander of the army for so many years, was grievously offended by it; and moreover, being a well-tried general, he had incomparably more distinction in the tribe of Judah than Amasa, who had taken part in Absalom’s rebellion and even led the rebel army, could possibly have.