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; Psalm 18:12-13; compare the notes at Isaiah 29:6.

Reserved against the time of trouble - לעת צר leeth tsar, "to the season of strictness," i.e., the season when the earth is constringed or bound by the frost.
Against the day of battle and war? - Hailstones being often employed as instruments of God's displeasure against his enemies, and the enemies of his people. There is probably an allusion here to the plague of hail sent on the Egyptians. See Exodus 9:23 (note), and the notes there, for more particulars concerning hailstones, remarkable showers of them, etc. There may be also a reference to Joshua 10:10-11 (note), where a destructive shower of what are called hailstones fell upon the Canaanitish kings who fought against Israel. See the note there also.

Which I have reserved (o) against the time of trouble, against the day of battle and war?
(o) To punish my enemies with them, (Exodus 9:18; Joshua 10:11).

Which I have reserved against the time of trouble,.... For the punishment or affliction of men; and is explained as follows,
against the day of battle and war? as his artillery and ammunition to light his enemies with. Of hail we have instances in Scripture, as employed against the Egyptians and Canaanites, Exodus 9:25; and of a reserve of it in the purposes of God, and in prophecy against the day of battle with antichrist, Revelation 16:21; and so Jarchi interprets it here of the war of Gog and Magog. And though there are no instances of snow being used in this way in Scripture, yet there is in history. Strabo (s) reports, that at Corzena and Cambysena, which join to Mount Caucasus, such snows have fallen, that whole companies of men have been swallowed up in them; and even armies have been overwhelmed with them, as the army of the Gauls (t); and such quantities have been thrown down from mountains, on which they have been lodged, that towns, towers, and villages, have been laid prostrate by them (u); and in the year 443, a vast snow destroyed many (w). Frequently do we hear in our parts of the disasters occasioned by them. The Targum particularly makes mention of snow; and renders it, "which snow I have reserved", &c. though absurdly applies it to punishment in hell.
(s) Geograph. l. 11. p. 363. (t) Cicero de Divinatione, l. 1. (u) Olaus Magu. de Ritu Gent. Septent. l. 2. c. 13. (w) Whiston's Chronolog. Tables, cent. 20.

against the time of trouble--the time when I design to chastise men (Exodus 9:18; Joshua 10:11; Revelation 16:21; Isaiah 28:17; Psalm 18:12-13; Haggai 2:17).

Trouble - When I intend to bring trouble upon any people for their sins.