As I've done with some of these short prophetic books, I want to share some of the background info I read:
When Jonah preached repentance on the streets of Nineveh, the capital of Assyria, the people responded and were spared. A century later (about 650 b.c.) Nahum preached in a time when Nineveh would not repent. Nineveh, which had destroyed Israel’s northern kingdom in 722, would itself fall to Babylon in 612—just a few years after Nahum’s warning. The Assyrians were notorious for the brutality of their treatment of other nations. Nahum declared, however, that God is sovereign: he punishes whom he will, and they are powerless to stop him. Much of Nahum’s prophecy was directed to the people of Judah, who could rejoice at the good news (1:15) of Nineveh’s impending fall.