Albert Barnes Commentary Job 38:23

"Which I have reserved against the time of trouble, Against the day of battle and war?" — Job 38:23 (ASV)

Which I have reserved - As if they were carefully treasured up to be brought forth as they will be needed. The idea is that they were entirely under the direction of God.

The time of trouble - Herder suggests “the time of need.” The meaning probably is that he had kept them in reserve for the time when he wished to bring calamity on his enemies, or that he made use of them to punish his enemies; compare the notes at Job 36:31-33.

Against the day of battle and war - Hailstones were sometimes employed by God to overwhelm his enemies and were sent against them in time of battle; see Joshua 10:11; Exodus 9:22–26; Psalms 18:12–13; compare the notes at Isaiah 29:6.