He teacheth my hands to war - Compare Psalm 144:1. The skill which David had in the use of the bow, the sword, or the spear - all of which depends on the hands - he ascribes entirely to God.
So that a bow of steel is broken by mine arms - This is mentioned as an instance of extraordinary strength, as if he were able to break a bow made of metal. The original word rendered steel means properly brass. Wood was doubtless first used in constructing the bow, but metals came afterward to be employed, and brass would naturally be used before the manufacture of steel was discovered. Rosenmuller in loc.

He teacheth my hands to war - The success which I have had in my military exercises I owe to the Divine help. How few of the conquerors of mankind can say so! And how few among those who call themselves Christian warriors dare to say so! War is as contrary to the spirit of Christianity as murder. Nothing can justify Christian nations in shedding each other's blood! All men should live in peace; all men might live in peace; and the nation that is first to break it is under a heavy curse.
A bow of steel is broken by mine arms - All the versions render this: "Thou hast made my arm like a brazen bow." A bow of steel is out of the question. In the days of David it is not likely that the method of making steel was known. The method of making brass out of copper was known at a very early period of the world; and the ancients had the art of hardening it, so as to work it into the most efficient swords. From his own account David was swift, courageous, and strong.

He teacheth my hands to war,.... From whence it appears, that war, in some cases, is lawful; and that all the skilfulness and art in training men for war, in the use of armour, in marshalling of armies, in forming sieges, &c. is all from God; see Psalm 144:1; and so is all that spiritual skill, in making use of the whole armour of God against every enemy, sin, Satan, and the world; and even the wisdom and skill, counsel and instruction, which Christ as man and Mediator had, when it was the hour and power of darkness; when he was engaged with principalities and powers, and got the victory over them, were from the Lord: see Psalm 16:7;
so that a bow of steel is broken by mine arms; that is, the bow of an enemy falling into his hands, which might be literally true of David. It is in the Hebrew text, "a bow of brass"; and so Apollinarius renders it; which Kimchi and Ben Melech interpret strong iron, that is, steel; and so the Targum; see Job 20:24; Satan is an archer; his temptations are darts, and fiery ones; and his strong bow may be said to be broken by the arms of faith, when his temptations, under the influence of divine grace, are repelled and rendered ineffectual; and especially his bow was broken by Christ, not only in the wilderness, when he was vanquished by him; but in the garden, and on the cross, when Satan could find nothing in him, and get no other advantage over him, but to bruise his heel; when he himself had his head broke, his works ruined, and he himself destroyed. Some render the words, "mine arms have bent a bow of steel": that is, such skill and strength were given him that he was able to bend, draw, and shoot a bow or steel: the Targum is, "and hath strengthened mine arm as a bow of brass", or "steel"; and so the Syriac and Arabic versions; and to the same purpose the Septuagint, Vulgate Latin, and Ethiopic versions; or it may be rendered, "my arms have bent", or "made to descend (o), a bow of brass"; for when a bow is bent, the horns or corners of it are made to descend towards a man.
(o) "ut current", Cocceius; so Michaelis; "ut descendat vel deprimatur", Gejerus; vid. Gussetii Comment. Ebr. p. 507. so Jarchi.