Nahum was the seventh of the so-called minor prophets. He prophesied during the same time as Habakkuk and Zephaniah. All we know of him is recorded in the book of his prophecies. He was probably a native of Galilee, and after the deportation of the ten tribes took up his residence in Jerusalem.
One hundred and fifty years before the time of Nahum, Jonah had prophesied against Ninevah. Because the people repented at the preaching of Jonah, the city was spared. However, they had eventually gone back to their old ways. Once again, God sent His prophet - this time Nahum - to warn them of His coming judgment. Once again, they did not listen.
As the book Nahum makes very clear, God will punish man for his wickedness. The Medes and Babylonians conquered Ninevah in 612 B.C. They destroyed it so completely that Alexander the Great stood on the site of its ruins three hundred years later and did not know it.