Jamieson, Fausset and Brown on 2 Samuel 20:4

4. Then said the king to Amasa, Assemble me the men of Judah within three days--Amasa is now installed in the command which David had promised him. The revolt of the ten tribes, probably, hastened the public declaration of this appointment, which he hoped would be popular with them, and Amasa was ordered within three days to levy a force from Judah sufficient to put down the insurrection. The appointment was a blunder, and the king soon perceived his error. The specified time passed, but Amasa could not muster the men. Dreading the loss of time, the king gave the commission to Abishai, and not to Joab--a new affront, which, no doubt, wounded the pride of the stern and haughty old general. But he hastened with his attached soldiers to go as second to his brother, determined to take the first opportunity of wreaking his vengeance on his successful rival.