King Nabonidus of Babylon was the father of his coregent, Belshazzar. He outlived his son, who was killed on the night that troops of Cyrus the Persian took Babylon, October 5, 539 B.C.E. (Dan. 5:30, 31)

In any event, secular history definitely establishes that Cyrus was a key figure in the conquest of Babylon and that he thereafter ruled there as king.