One of Joab’s young men stood near Amasa and said, “Whoever favors Joab, and whoever is for David, let him follow Joab!”
We have here Amasa's fall just as he began to rise. He was nephew to David (Sa2 17:25), had been Absalom's general and commander-in-chief of his rebellious army, but, that being routed, he came over into David's interest, upon a promise that he should be general of his forces instead of Joab.
Commenting on 2 Samuel 20:4-13
And one of Joab's men stood by him,.... By the body of Amasa; no doubt by the order of Joab, to satisfy the people as they came up, and reconcile them to this fact, and to exhort them not to stop, but to follow after Joab; for though Amasa their general was dead, Joab had taken the command of the army, and the pursuit was...
He that favoureth Joab, and he that is for David, let him go after Joab--It is a striking proof of Joab's unrivalled influence over the army, that with this villainous murder perpetrated before their eyes they unanimously followed him as their leader in pursuit of Sheba.