After freeing himself from Scythian domination, Cyaxares embarked on reorganizing and strengthening the Median army. He expanded his capital, Ecbatana, and reinforced its military fortifications. At this time, fortune favored him. Ashurbanipal, the bloodthirsty king of Assyria, died in 627 BCE, and after him, Sîn-šar-iškun, his weaker and less capable successor, ascended the throne. This change in Assyria's leadership provided Cyaxares with a golden opportunity to resume his plans for destroying the oppressive empire and seeking long-awaited revenge.
Concurrent with Assyria's weakening, Nabopolassar, the Chaldean commander and Assyria's vassal in Babylon, rebelled against his overlords and founded the Neo-Babylonian kingdom in 626 BCE. Nabopolassar allied with Cyaxares, and this alliance was cemented through the marriage of Cyaxares's daughter, Amytis, to Nabopolassar's son, Nebuchadnezzar II, who later became a powerful king of Babylon. This marriage not only strengthened political and military ties between Media and Babylon but also provided a more solid foundation for joint resistance against Assyrian threats, mobilizing the forces of the two great regional powers against their common enemy.
A bloody war ensued. Nabopolassar managed to defeat the Assyrian forces at the Battle of Gablin in 616 BCE. The following year, the Babylonians achieved another victory, defeating the Assyrian forces in Arabkha and reaching the city of Assur. They besieged the city but failed to capture it. This demonstrated the resistance of the ancient Assyrian capital.
In 615 BCE, Cyaxares also launched an assault on Arabkha and achieved a valuable victory. He then attacked the city of Assur and succeeded in capturing the ancient capital of the Assyrians. Following the custom of his time, he set the entire city ablaze, and with this event, nothing but ruins remained of the once magnificent city of Assur. After this, the Medes attacked Nineveh but failed to capture the city. It was at this time that Cyaxares and Nabopolassar forged a bond of brotherhood in the ruins of the city of Assur.
Finally, in 612 BCE, the united forces of Media and Babylon reached the walls of Nineveh. The siege lasted three months, and ultimately, by creating an artificial flood, they managed to breach a section of the eastern wall, thus infiltrating the city. Sîn-šar-iškun, the king of Assyria, seeing his city burn, threw himself into the flames out of despair. The ancient Assyrian Empire, amidst fire and blood, having given nothing else to the world, was destroyed forever.
After the fall of Nineveh, Sîn-šar-iškun's brother, Ashur-uballit II, reorganized the remaining forces and fled Nineveh. Ashur-uballit took refuge in Harran with the remnants of the Assyrian forces. The united Median and Babylonian forces, after plundering Nineveh and utterly destroying it, pursued Ashur-uballit. Another battle took place between the two sides in Harran, where the allies again emerged victorious. Ashur-uballit took refuge in Carchemish and requested aid from the king of Urartu. Ultimately, with the defeat of Ashur-uballit in 605 BCE, the powerful Assyrian Empire officially came to an end.
With the conquest of Nineveh, Cyaxares became the unrivaled ruler of the Eastern world. The Median territory expanded from Khorasan in the east to Asia Minor in the west, becoming the largest empire in the region. But this was not the end of his work. He became neighbors with the wealthy state of Lydia, in the region that is now Turkey. Alyattes, the king of Lydia, refused to hand over the fugitive Scythians to Cyaxares, initiating a long war that lasted five or six years, with neither side able to achieve a decisive victory. In 585 BCE, during the battle, a great solar eclipse occurred. Both sides regarded this phenomenon as a sign of divine wrath, and the war ended in peace. Alyattes gave his daughter in marriage to Astyages, Cyaxares's son, and a lasting peace was established. A bas-relief in Persepolis depicts a Median nobleman hand-in-hand with a Lydian, walking towards Cyrus's court; a symbol of the end of this conflict and the beginning of peaceful relations. Cyaxares, who had previously conquered the Mannaean state and the kingdom of Urartu, died in the year following this peace. He left behind a vast territory stretching from Khorasan to the Halys River. His victory over Assyria proved to the world that the power of the Semitic people was over, and from then on, the Iranian Aryans would be the masters of Asia.
Cyaxares, by conquering Nineveh, not only changed the map of Asia but also ushered history into a new phase. He rekindled hope for the Medes and was rightly named the greatest king of Media and the founder of the Aryans' presence in world history. His story was one of courage, prudence, and ambition that paved the way for the emergence of the Persian Empire of Cyrus the Great, his maternal grandson. These victories and organizations became a legacy that Cyrus the Great later brought to its peak with his modern knowledge.