What does Isaiah 13:9 mean?
The "day of the Lord" has come. In Old Testament prophecy, this phrase carries enormous weight. It usually indicates a time of great terror and suffering as God unleashes His wrath against human sinfulness (Ezekiel 30:3; Joel 1:15; Obadiah 1:15). In this case, the day of the Lord has come for the proud nation of Babylon (Isaiah 13:1).This judgment coming for Babylon is described using a Hebrew word, often translated as "cruel," which carries the idea of something fierce and ruthless. This is also tied to God's holy anger. This is an uncomfortable description, and one of the only times in Scripture where the Lord's own actions are designated that way. Modern language often associates cruelty with sadism: a deliberate attempt to cause pain. However, the main meaning is the opposite of mercy: this is an unchanging, unrelenting, absolute form of judgment. This is even more terrifying because it is always just. It is always in proportion to human sinfulness. It delivers the fierce punishment our sin deserves for violating the commands of the Lord, as well as worshiping false versions of His divine nature. When human beings are cruel, we forget our own limitations and imperfections—God has no such flaws, so what might be "cruel" for mankind is the result of holy anger when applied by God.
In this case, the wrath of God has assembled armies from multiple nations that He will use to make the land of Babylon a wasteland (Isaiah 13:3–5). He is preparing to destroy all the sinners in the land, leaving nothing behind (Isaiah 13:6).