The main theme of Nahum is that the Lord is going to destroy Nineveh – the capital city of the Assyrians. This is in dramatic contrast to the book of Jonah.
About 100 years earlier the Lord had sent Jonah to call on Ninevah to repent – which they did. Clearly this has not lasted and future generations have rejected the Lord and become the most feared and ruthless enemy of his people.
1 An oracle concerning Nineveh. The book of the vision of Nahum of Elkosh.