Charles Ellicott Commentary 2 Chronicles 25:5

"Moreover Amaziah gathered Judah together, and ordered them according to their fathers` houses, under captains of thousands and captains of hundreds, even all Judah and Benjamin: and he numbered them from twenty years old and upward, and found them three hundred thousand chosen men, able to go forth to war, that could handle spear and shield." — 2 Chronicles 25:5 (ASV)

AMAZIAH’S MILITARY STRENGTH, AND CONQUEST OF EDOM
(2 Chronicles 25:5–13).

This section is for the most part peculiar to Chronicles. In Kings the conquest of Edom is recorded in a single verse (2 Kings 14:7).

And made them captains over thousands. —Rather, And made them stand (marshalled them) according to father houses, namely, according to the captains of thousands and according to the captains of hundreds of all Judah and Benjamin.

Numbered. — Mustered.

Twenty years old. —The military age: Numbers 1:2–3; 1 Chronicles 27:23.

Three hundred thousand. —A total immensely below that of the forces of Jehoshaphat (2 Chronicles 17:14–18), and not much more than half that of Asa’s (2 Chronicles 14:8). All these high numbers are no doubt suspicious; but a certain relative propriety is observable in the present instance, inasmuch as the country had suffered great losses by the disastrous wars of Jehoram, Ahaziah, Joash.

Able to go forth to war. —Literally, going out in the host (See Numbers, in the place cited).

That could handle spear and shield. — Grasping lance and target, i.e., heavy-armed warriors (Compare to 1 Chronicles 12:8).