Which I have reserved - As if they were carefully treasured up to be brought forth as they shall be needed. The idea is, that they were entirely under the direction of God.
The time of trouble - Herder “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 foes; compare the notes at Job 36:31-33.
Against the day of battle and war - Hailstones were employed by God sometimes to overwhelm his foes, 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.