After freeing himself from Scythian domination, Cyaxares focused 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, his weaker and less competent successor, Sin-shar-ishkun, ascended the throne. This change in Assyrian leadership provided a golden opportunity for Cyaxares to resume his plans for the destruction of this tyrannical empire and to seek long-awaited revenge.
Simultaneously with Assyria's weakness, Nabopolassar, a Chaldean commander and a vassal of Assyria in Babylon, rebelled against his masters and founded the Neo-Babylonian Empire in 626 BCE. Nabopolassar allied with Cyaxares, and this alliance was sealed through the marriage of Cyaxares's daughter, Amyitis, to Nabopolassar's son, Nebuchadnezzar II, who would later become the powerful king of Babylon. This marriage not only helped to strengthen the political and military relations between Media and Babylon but also provided a more solid foundation for joint resistance against the Assyrian threat, mobilizing the forces of the two great regional powers against their common enemy.
A bloody war ensued. In 616 BCE, Nabopolassar managed to defeat the Assyrian forces in the Battle of Gablin. The following year, the Babylonians achieved another victory, defeating the Assyrian forces at Arrapha and reaching the city of Ashur. They laid siege to the city but were unsuccessful in capturing it. This demonstrated the resistance of the ancient Assyrian capital.
In 615 BCE, Cyaxares also attacked Arrapha and achieved a valuable victory. He then attacked the city of Ashur and succeeded in capturing the ancient capital of the Assyrians. In accordance with the customs of his time, he set the entire city on fire, and with this event, nothing remained of the glorious city of Ashur but ruins. After this, the Medes marched on Nineveh but were unsuccessful in capturing the city. It was at this time that Cyaxares and Nabopolassar made a pact of brotherhood in the ruins of the city of Ashur.
Finally, in 612 BCE, the allied forces of Media and Babylon reached the walls of Nineveh. The siege lasted for three months, and in the end, by creating an artificial flood, they were able to destroy a part of the eastern wall of the city and thus penetrate it. Sin-shar-ishkun, the king of Assyria, seeing his city burn, threw himself into the flames in despair. The long-standing Assyrian Empire, amidst the fire and blood that it had gifted the world, was destroyed forever.
After the fall of Nineveh, Sin-shar-ishkun's brother, Ashur-uballit II, organized the remaining forces and fled from Nineveh. Ashur-uballit took refuge in Harran with the remnants of the Assyrian forces. The allied forces of Media and Babylon, after plundering and completely destroying the city of Nineveh, pursued Ashur-uballit. Another battle took place in Harran, in which the allies were once again victorious. Ashur-uballit fled to Carchemish and requested help from the king of Urartu. Ultimately, with the defeat of Ashur-uballit in 605 BCE, the powerful Assyrian Empire and the kingdom of Urartu officially came to an end.
With the conquest of Nineveh, Cyaxares became the undisputed ruler of the eastern world. The territory of the Medes 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. He became neighbors with the wealthy state of Lydia, in the region of modern-day Turkey. Alyattes, the king of Lydia, refused to hand over fugitive Scythians to Cyaxares, and this began a long war that lasted for five or six years, with neither side able to achieve a decisive victory. In 585 BCE, in the midst of a battle, a great solar eclipse occurred. Both sides saw this phenomenon as a sign of the gods' wrath, and the war ended in peace. Alyattes gave his daughter in marriage to Astyages, the son of Cyaxares, and a lasting peace was established. A relief in Persepolis shows a Median nobleman holding hands with a Lydian, walking towards the court of Cyrus, 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 a year after this peace. He left behind a vast territory from Khorasan to the Halys River. His victory over Assyria proved to the world that the power of the Semitic people had come to an end, and from now on, the Iranian Aryans would be the masters of Asia.
Cyaxares, with the conquest of Nineveh, not only changed the map of Asia but also moved the course of history to a new stage. He revived the hope of the Medes and is rightly called the greatest king of Media and the founder of the Aryan 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 would later, with his modern knowledge, bring to its zenith.