Albert Barnes Commentary Isaiah 30:31

"For through the voice of Jehovah shall the Assyrian be dismayed; with his rod will he smite [him]." — Isaiah 30:31 (ASV)

For through the voice of the—Lord, by the command of the Lord; that is, His voice going forth in the manner specified in (Isaiah 30:30).

Which smote with a rod—Who was accustomed to smite as with a rod; that is, his government was tyrannical and severe. As he had been accustomed to smite in that manner, so he would now meet the proper reward of his oppression of the nations.