Charles Ellicott Commentary 2 Kings 3:21

"Now when all the Moabites heard that the kings were come up to fight against them, they gathered themselves together, all that were able to put on armor, and upward, and stood on the border." — 2 Kings 3:21 (ASV)

And when ... heard ... they gathered. — Now all the Moabites had heard ... and had gathered themselves:

Literally, had been summoned, called together (Judges 7:23).

All that were able to put on armour. — From every one girding on a girdle, and upwards — i.e., all of adult age, all who could bear arms. It was a levy en masse of the male population for the defence of the country.

Stood in. — Had taken their stand on the frontier.