Then the king said to Amasa, “Summon the men of Judah to come to me within three days, and be here yourself.”
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
Then said the king to Amasa,.... Whom he had promised to make general of his army, Sa2 19:13; and by the following order declared him such: assemble me the men of Judah within three days; which was done by the sound of the trumpet, or by the proclamation of a herald; it seems that the men of Judah, who attended David to Jerusalem, were gone...
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.