We can date Nahum to sometime after Assyria conquered the Egyptian city of Thebes (663 BC) and before Assyria was itself conquered by Babylon (612 BC). The weakening Assyrian Empire had wreaked havoc for over a hundred years as the most vicious military force ever known.
Not content to conquer, Assyria subdued her enemies by exiling them to the far reaches of the empire so that they could never again rally an army and fight back. Thus in 722 BC, Samaria (the capital of Israel - the northern kingdom) was conquered and its people relocated, never to be heard from again. For the next century, Jerusalem (the capital of Judah - the southern kingdom) lived in fear it would be the next to suffer Assyria's wrath.
Nahum speaks out against the aggressor empire, boldly proclaiming that Assyria would get a taste of its own medicine and that one day soon Nineveh (the empire's capital) would be destroyed. The Lord is good, a refuge in times of trouble. He cares for those who trust in him, but with an overwhelming flood he will make an end of Nineveh (1:7,8).
The prophet concludes by saying that no one will shed a tear for Assyria. No one will mourn for her, and no one will offer her comfort (3:7). Rather, there will be rejoicing at the news of her collapse.
What will people say about you after you're gone?