Let not the swift flee away. A strong way of expressing that even the swiftest cannot expect to flee, just as, in Isaiah 2:9, "forgive them not" means "thou canst not forgive them." Nothing seems to have struck the Jews so much as the unparalleled swiftness of the Chaldean warriors (Hebrews 1:6, Hebrews 1:8; Jeremiah 4:13).
They shall stumble; literally, they have stumbled; it is most probably the prophetic perfect ("they shall certainly fall"), though Ewald denies this, and consequently maintains that the prophecy was written after the battle of Carchemish.
Toward the north; i.e. "in the northern region," or, more loosely, "in the north" (comp. Isaiah 2:10). Carchemish was, of course, far to the north of Jerusalem.