A rod may refer to a royal sceptre (see Psalm 45:6; Zechariah 10:1) or an object with which someone is punished (see Proverbs 13:24;Proverbs 26:3). In this context, both these meanings are combined.1 God, the royal judge, is punishing his people. A reinforcement of this idea is to be found in Isaiah 10:5: Woe to Assyria, the rod of my anger; the staff in their hands is my fury!
(Other prophets also use this image in reference to God's use of an unbelieving nation to punish the Israelites.) The image further demonstrates God’s direct action in punishing his people, although he uses others for it. What should also be noted is that it is not so much the anger of God that is depicted here as the fact of his judgment, though it is fierce.2
1 Je suis l'homme qui a vu la misère Sous la verge de sa fureur.