Can a leopard change its spots?

"Woe to the city of blood, full of lies, full of plunder, never without victims!" Nahum 3.1

it seems that Jonah's impact on Nineveh, though effective at the time, did not last. The Assyrians, whose capital was Nineveh, "enjoyed" a reputation for merciless cruelty and ruthlessness. No wonder Jonah was reluctant! Though God may have used them to bring judgment on the northern kingdom of Israel, their own wickedness invited judgment, as Nahum confirms in his prophecy which contains a vision of Nineveh's come-uppance and - with it - the destruction of the Assyrian Empire, which occurred at the hands of the Babylonians in 612 BC.

Nahum means "comfort" and his message was intended to comfort the victims of Assyria, in particular, that they would not suffer indefinitely. More generally, his message reassures all victims of oppression that evil cannot prevail. While God, in his mercy and grace, is slow to anger, those same qualities render him swift to save and rescue.