The entire book of Nahum is dedicated to the eventual destruction of Nineveh.  While it's been destroyed on several occasions, it's always been rebuilt.  However, this time, when this prophecy sees its final fulfillment, Nineveh (modern day Mosul) will be completely destroyed once and for all, never to rise again or ever be rebuilt.  Once it is destroyed this one final time, likely sometime in the near future, and potentially connected or related to the destruction of Damascus and Syria, it will never, ever rise again for the rest of eternity.  At that point its judgment will be final.