Isaiah 31:8 - Ellicott's Commentary On The Whole Bible

Not of a mighty man... — The Hebrew has no adjectives, but the nouns are those which are commonly opposed to each other in this way, as in Isaiah 2:9, like the Latin vir and homo. The thought expressed is, of course, that the whole work would be of God, and not of man. The “sword” was that of the Divine judgment (Deuteronomy 32:41), perhaps, as in 1 Chronicles 21:16, of the destroying angel of the pestilence.