Kepler, I have noticed that Bible writers use the title king very loosely, e.g., Nebuchadnezzar is called king even though he was not yet king (Dan. 1:1). Same goes for Belshazzar, he is addressed as king, but as seen he was subservient to his father Nabonidus (Dan. 5:18).
Just a few thoughts on Nabonidus. He spent at least 10 years of his seventeen years in Teima, Arabia. He was highly unpopular because of forsaking Marduk, one of Babylon’s most prominent deities for Sin, the moon god. He was involved with building projects at Teima and Haran, not Babylon. There is good reason to suspect that Belshazzar’s feast was an akitu festival in honor of Sin, the moon god. So Belshazzar was not very popular either. The following excerpt by Montgomery explains the relationship of Nabonidus and Belshazzar:
In the cuneiform texts Belsh. is called either by his name or, as in the Nabonidus-Cyrus Chronicle simply ‘son of the king,’ i.e., anglice, ‘crown prince.’ In the Chronicle for years 7, 9, 10, 11 of Nabonidus’ reign it is recorded that “the king was in Teima; the son of the king, the princes and his (or, the) army were in the land of Akkad.” In the texts hitherto known Belsh. is never given the title of king, and this has been ground for argument against one detail of our story which represents Belsh. as absolute king. But Sidney Smith’s presentation of a new text (s. end of Note 12) shows that royal dignity was actually conferred upon Belsh. This text, of the third full year of Nabonidus, detailing that king’s victorious campaign against Arabian Teima (as this place has elsewhere been identified by Dougherty), records: “He intrusted a camp to his eldest, his first-born son; the troops of the land he sent with him. He freed his hand; he intrusted the kingship (šarrûtam) to him.” That is, in the early part of Nabonidus’ reign, in his third year, his son was invested with royal dignity, which, in view of the active position he held throughout the subsequent years, must have continued throughout his life. That is, the Bible story is correct as to the rank of kingship given to Belsh. Now in several texts the prince’s name is coupled with his father’s in the latter’s prayers and in the omens interpreted for him; and in Pinches’ text and two texts in the Yale Museum his name is associated with his father’s in an oath; on which Dougherty remarks: “There is no other instance in available documents of an oath being sworn in the name of the son of the king.” The induction therefore that had been made from earlier data by Pinches, Dougherty, and others, is now brilliantly corroborated; as in a previous statement of the latter scholar: “It appears that he was invested with a degree of royal authority, not only at the close of the reign of his father, but throughout large part, if not the whole, of the reign of Nabonidus.”[1]
And now one can understand the motives of the writer of the Cyrus Cylinder. He hated Nabonidus, and probably was an adherent of Marduk. Cyrus, again, was a popular monarch because he was religiously tolerant.
Eventually and inevitably the truth will become known. I won’t write off the evidence of the Bible writers as yet. Sometimes these are biased and slant things according to the Jewish perspective. Nevertheless, truth can be found in the Bible accounts, but it takes effort to unearth it. E.g., if one compares the Babylonian Chronology of the Babylonian Chronicles to that of the Bible account, there are few discrepancies. These are mainly differences in regnal/ascencion years, different calenders, etc. My favorite saying will always be: Where there’s smoke there’s fire. By hook or by crook, the truth will come out.
[1] Montgomery, J. A. (1927). A critical and exegetical commentary on the book of Daniel (pp. 66, 67). New York: Charles Scribner’s Sons.