When they were at the great stone that is in Gibeon, Amasa came to meet them. Now Joab was wearing a soldier’s garment, and over it was a belt with a sword in its sheath fastened on his thigh, and as he went forward it fell out.
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
When they were at the great stone which is in Gibeon,.... Which, according to Josephus (r), was forty furlongs, or five miles from Jerusalem: what this great stone was, whether an obelisk, or what, is not certain; one of the greatest stones we read of was that which Semiramis cut out of the mountains of Armenia, which was an hundred thirty feet long, and twenty five broad and thick (s).
Amasa went before them--Having collected some forces, he by a rapid march overtook the expedition at Gibeon, and assumed the place of commander; in which capacity, he was saluted, among others, by Joab. Joab's garment, that he had put on was girded unto him--in the fashion of travellers and soldiers. a sword . . . and as he went forth it fell out--that is, out of the scabbard.