What does Proverbs 21:31 mean?

The prior proverb noted, broadly, that no human effort could possibly undo God's intended purposes. This verse underscores the Lord's sovereign control of history. A nation may mount an impressive arsenal of weapons, but victory does not depend exclusively upon weapons. God's will is infinitely more powerful and is ultimately all that matters.

King David understood that chariots and horses—the pinnacle of military technology in that era—do not guarantee victory. He writes in Psalm 20:7–8: "Some trust in chariots and some in horses, but we trust in the name of the LORD our God. They collapse and fall, but we rise and stand upright." He also writes in Psalm 33:17: "The war horse is a false hope for salvation, and by its great might it cannot rescue." A nation's success in battle is entirely dependent upon the Lord's will.