Question: "When and how was Nineveh destroyed?"
Answer: Nineveh, the capital of Assyria, was destroyed in 612 B.C. by the Medes. This was in fulfillment of the prophet Nahum’s prediction that God would completely destroy the city (Nahum 1). A number of factors combine to determine both the date and manner of Nineveh’s destruction.
During the prophet Jonah’s day, Nineveh was spared by God’s compassion in response to their repentance (Jonah 3). This happened in 760 B.C.
The book of Nahum was written after the destruction of the Egyptian city of Thebes (Nahum 3:8). That event took place in 663 B.C. when it was conquered by the Assyrian king Ashurbanipal. Therefore, Nineveh was still standing at that time. There is some evidence that Nahum wrote shortly after the destruction of Thebes, because Judah was still under Assyrian control during the time of his writing. This was the situation during the reign of Manasseh (697-642 B.C.) but not during the reign of Josiah (640-609 B.C.). In addition, the city of Thebes returned to power in 654 B.C., meaning that Nahum likely wrote before then. So, Nahum can be dated between 663 and 654 B.C. Therefore, Nineveh must have been destroyed after 654 B.C. but no later than 612, when the Medes are mentioned as the conquerors of the city.
An ancient account called The Fall of Nineveh Chronicle reveals an account of this time period, providing firsthand, extra-biblical documentation. The translation (with some missing text) reads as follows:
“The king of Akkad mustered his army and marched to Assyria. The king of the Medes marched towards the king of Akkad and they met one another at [...]u. The king of Akkad and his army crossed the Tigris; Cyaxares had to cross the Radanu, and they marched along the bank of the Tigris. In the month Simanu [May/June], the Nth day, they encamped against Nineveh.
“From the month Simanu until the month Âbu [July/August] -for three months- they subjected the city to a heavy siege. On the Nth day of the month Âbu they inflicted a major defeat upon a great people. At that time Sin-šar-iškun, king of Assyria, died. They carried off the vast booty of the city and the temple and turned the city into a ruin heap The [lacuna] of Assyria escaped from the enemy and, to save his life, seized the feet of the king of Akkad.
“On the twentieth day of the month Ulűlu [14 September 612] Cyaxares and his army went home.” (From http://www.livius.org/ne-nn/nineveh/nineveh02.html#Fall.)
Based on this account, it is clear that the siege of Nineveh came at the hands of the king of Akkad and the king of Media during the summer of 612 B.C. Three months later, the city fell. The king of Assyria died, and the city was plundered until September 14 when the invading army departed. By 605 B.C. the Assyrian Kingdom officially ended, and Babylonia was on the rise.
Despite Nineveh’s great power, the city fell just as Nahum had prophesied. It would not be until the 1800s that archaeologists would excavate portions of the ancient city. Nineveh had indeed been “hidden,” as Nahum predicted long ago (Nahum 3:11).