Charles Ellicott Commentary Isaiah 31:8

"And the Assyrian shall fall by the sword, not of man; and the sword, not of men, shall devour him; and he shall flee from the sword, and his young men shall become subject to taskwork." — Isaiah 31:8 (ASV)

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.