Whose arrows are sharp - Bows and arrows were the common instruments of fighting at a distance. Arrows were, of course, made sharp, and usually pointed with iron, for the purpose of penetrating the shields or coats of mail which were used to guard against them.
And all their bows bent - All ready for battle.
Their horses’ hoofs shall be counted like flint - It is supposed that the ancients did not usually shoe their horses. Hence, a hard, solid hoof would add greatly to the value of a horse. The prophet here means, that their horses would be prepared for any fatigue, or any expedition; see a full description of horses and chariots in Bochart’s “Hieroz.” P. i. lib. ii. ch. viii. ix.
And their wheels like a whirlwind - That is, the wheels of their chariots shall be swift as the wind, and they shall raise a cloud of dust like a whirlwind. This comparison was very common, as it is now; see “Bochart.” See, also, a magnificent description of a war-horse in Job 39:19-25.