Cyaxares collaborated with the Babylonians to destroy the Assyrian Empire, and united most of the Iranian peoples of ancient Iran, thereby transforming Media into a major power.

[4][19][16] After freeing the Medes from the Scythian yoke, Cyaxares reorganised the Median armed forces in preparation for a war with Assyria: whereas the Medes previously fought as tribal militias divided into kinship groups and each warrior used whatever weapons they were the most skilled at, Cyaxares instituted a regular army modelled on the Assyrian and Urartian armies, fully equipped by the state and divided into strategic and tactical units.

[10] Following the defeat of a joint Assyrian-Mannaean force at Gablinu by the new Babylonian rebel king and founder of the Neo-Babylonian Empire, Nabopolassar, the next year Cyaxares conquered Mannae, which brought the Median armies to the frontiers of Assyria.

[4] In November 615 BCE, six months after Nabopolassar had failed to seize the important Assyrian centre of Assur, Cyaxares crossed the Zagros mountains and occupied the city of Arrapha.

In 612 BCE, the Median and Babylonian armies together crossed the ʿAdhaim river at its mouth and marched on the Assyrian capital city, Nineveh, which was taken and sacked by the joint Medo-Babylonian forces after three months of siege.

In 610 BCE, Cyaxares and Nabopolassar seized Harran from the Assyro-Egyptian force, which retreated to Carchemish on the west bank of the Euphrates.

[28] According to older interpretations of the destruction of the Neo-Assyrian Empire, its territory was partitioned between the Babylonians and the Medes, the latter of whom obtained a territory which included Assyria proper and had a southern border which started at Carchemish and passed south of Harran and along the Jabal Sinjār till the Tigris to the south of Assur, and then along the Jabāl Hamrīn and across the Diyala River valley until the northwestern borders of Elam.

[29] Following the destruction of the Assyrian Empire, the majority of the Scythians were expelled frmo Western Asia and into the Pontic Steppe during the 600s BCE,[21] and the relations between the Medes and the Babylonians soon temporarily deteriorated in the 590s.

The kings of Babylon and Cilicia acted as mediators in the ensuing peace treaty, which was sealed by the marriage of Cyaxares's son Astyages with Alyattes's daughter Aryenis.

The Russian historian Igor Diakonoff has tentatively suggested that the tomb of Cyaxares might be located at the place now called Qyzqapan, in the mountains of present-day Iraqi Kurdistan in Sulaymaniyah.

After these were defeated, Darius noted two in the Behistun Inscription: "Another was Phraortes, the Mede; he lied, saying: 'I am Khshathrita, of the dynasty of Cyaxares.'