The book of Nahum in the Old Testament is a book of prophecy of God's judgments upon Nineveh.
The prophet Nahum prophesied in Judah sometime between 663-612 B.C. His entire written message prophesies of the destruction of Nineveh, the capital city of Assyria.
The Assyrians conquered the northern kingdom of Israel about 721 B.C. and carried its people into custody. From that time on the Assyrians were a steady threat to the survival of Judah as well. Nahum's prophecy of the destruction of Assyria, written in Hebrew poetry, must have been a source of hope and comfort to the people of Judah.
Nineveh had received a warning to repent through the prophet Jonah more than one hundred years earlier, in which they did repent and were spared by the Lord. However, by the time Nahum's ministry, Nineveh had become wicked again and this time would not escape the Lord's judgment.
Nahum's prophecies about the destruction of Assyria are relevant to the world today and at the time of the Second Coming.