Cyaxares II was said to be a king of the Medes

Medes

The MedesThe Medes...

 whose reign is described by the Greek historian Xenophon

Xenophon

Xenophon , son of Gryllus, of the deme Erchia of Athens, also known as Xenophon of Athens, was a Greek historian, soldier, mercenary, philosopher and a contemporary and admirer of Socrates...

. Some theories have equated this figure with the "Darius the Mede

Darius the Mede

Darius the Mede is a biblical person in the Book of Daniel, Chapters 6-9, who rules over Babylon after King Belshazzar is deposed. The author of the book of Daniel indicated that Darius was about 62 years old when he was 'made king over Babylon.[5:31] He is best known for having been forced into...

" named in the Book of Daniel

Book of Daniel

The Book of Daniel is a book in the Hebrew Bible. The book tells of how Daniel, and his Judean companions, were inducted into Babylon during Jewish exile, and how their positions elevated in the court of Nebuchadnezzar. The court tales span events that occur during the reigns of Nebuchadnezzar,...

. However, the fact that he is not mentioned at all in the history of Herodotus

Herodotus

Herodotus was an ancient Greek historian who was born in Halicarnassus, Caria and lived in the 5th century BC . He has been called the "Father of History", and was the first historian known to collect his materials systematically, test their accuracy to a certain extent and arrange them in a...

, nor in the very different history of Ctesias

Ctesias

Ctesias of Cnidus was a Greek physician and historian from Cnidus in Caria. Ctesias, who lived in the 5th century BC, was physician to Artaxerxes Mnemon, whom he accompanied in 401 BC on his expedition against his brother Cyrus the Younger....

, has caused many scholars to debate whether such a king ever actually existed.

According to Xenophon's Cyropaedia, Cyaxares II followed king Astyages

Astyages

Astyages Astyages Astyages (spelled by Herodotus as Ἀστυάγης - Astyages; by Ctesias as Astyigas; by Diodorus as Aspadas; Akkadian: Ištumegu, was the last king of the Median Empire, r...

 to the throne of the Mede Empire, and was also brother of Mandane, Cyrus the Great

Cyrus the Great

Cyrus II of Persia , commonly known as Cyrus the Great, also known as Cyrus the Elder, was the founder of the Achaemenid Empire. Under his rule, the empire embraced all the previous civilized states of the ancient Near East, expanded vastly and eventually conquered most of Southwest Asia and much...

's mother. He describes the Persian Cyrus as cooperating with his uncle, Cyaxares, in order to conquer Babylon

Babylon

Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

in 539 BC. However Cyaxares was by then an old man, and because Cyrus was in command of the campaign, the army came to regard Cyrus as king. Cyrus thus received not only the king's daughter (his first cousin), but his kingdom, as dowry, and the aged Cyaxares became Cyrus' viceroy in Babylon for two years until his death, when Cyrus seized that kingdom as well.