Historians point to the sudden silence in Assyrian history as the period in which the fate of the Empire was determined, with the rise of Scythian tribes and their reign of terror being signs of the Empire's eventual downfall.
The city of Nineveh is attacked by the combined forces of the Medes, Babylonians and a horde of Scythians, after the Assyrians were weakened by a civil war, but despite putting up a good fight, the city eventually falls due to its massive fortifications being breached.
The fall of the Assyrian Empire was marked with anger and vengeance from the people. Soldiers were motivated by more than just taking loot and slaves, as demonstrated by the defacing of Assyrian reliefs in the British Museum today.