Battle of Hyrba
The Battle of Hyrba was the first battle between the Persians and Medians.
It was also the first battle after the Persians had revolted. These actions were led (for the most part) by Cyrus the Great, as it shifted the powers of the ancient Middle East. The success of the battle led to the creation of Persia's first empire, and began Cyrus's decade long conquest of almost all of the known world. Though the only authority with a detailed account of the battle was Nicolaus of Damascus, other well known historians as Herodotus, Ctesias, and Strabo also mention the battle in their own accounts.
The outcome of the battle was such a great blow to Medes, that Astyages decided to personally invade Persia. The hasty invasion would eventually lead to his downfall. In turn, the former enemies of the Medes tried to move against them, only to be stopped by Cyrus. Thus a period of reconciliation began, which facilitated a close relationship between the Persians and Medes, and enabled Ecbatana, capital of Media, to pass to the Persians as one of Persia's capitals in their newly formed empire.