Nahum Chapter 1:8

This study was done using the Amplified Bible, which I don't have permission to publish, so I will replace the verses here with the American Standard Version.  Because of that some of the comments may not totally make sense because the AMP has expanded explanations.  I would highly recommend going to a site like BibleGateway.com where you can read the AMP version for free.

Nahum Chapter 1:8 But with an over-running flood he will make a full end of her place, and will pursue his enemies into darkness.

A. The city of Nineveh (located near modern day Mosul, Iraq) was the capitol of Assyria, a nation that has a large role in Scripture, and a long time rival of Babylonia until a joint effort between the armies of the Medes, Persians, Chaldeans, Scythians, Cimmerians and Babylonians approximately 612 BC destroyed it and subsequently destroyed the rest of the Neo-Assyrian empire.  By this incredible flood of enemy forces, the analogy of an overrunning flood definitively destroying Nineveh was fulfilled.

B.  It is generally accepted that Nahum was active somewhere between the years of 663 and 654 BC, which would mean he prophesied of Nineveh's destruction between 40-50 years before the event took place.  Most Christians today call someone a false prophet if the prophecy doesn't take place within a few weeks, maybe up to a year even, but we need to realize that most of the prophecies in the Bible weren't fulfilled for a significant amount of time, and now this lends a lot of credence to their claims.  When Nahum made this prophecy  Assyria was ruled by Ashurbanipal (669–627 BC and was easily one of the most powerful, if not the most powerful nation in the world. 

       Ashurbanipal was a brilliant man who not only squashed Elam, a lingering enemy of the Assyrian empire, defeated his own brother's rebellion who was the Assyrian king of Babylonia at the time, and founded the first library in history to organize books by genre.  The famous Library of Ashurbanipal was huge, containing works from all over Mesopotamia.

All of this being said, to prophecy that Nineveh would be destroyed at the time he prophesied it must have sounded insane to most people.