He teacheth my hands to war - Compare Psalms 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.