Chapter 5 Belshazzar's Feast.
When Nebuchadnezzar died he was succeeded by his son Amel Marduk (Evil-Merodach - 2 Kings 25:27-30), who was then succeeded within two years by Nergal-shar-usur (Jeremiah 39:3; Jeremiah 39:13), Nebuchadnezzar's son-in-law. He only survived for four years and died leaving on the throne a son, who was a minor, Labashi-Marduk, and within a short while this son had been replaced by Nabonidus, possibly the scion of a noble family of Aramaean stock in Haran, who seized the throne with the help of disaffected people and cemented his position by marrying the daughter of Nebuchadnezzar, who bore him a son named Bel-shar-usur (Belshazzar).
Nabonidus eventually left his son Belshazzar, Nebuchadnezzar's grandson, to hold the reins of kingship, first in order to conduct campaigns elsewhere, including Arabia, and then in order to spend his time in the city of Teima in Arabia possibly pursuing the study of astrology. He also refused to pay due deference to Marduk, absenting himself for long periods from the Babylonian akitu festival, to the anger of the priests who certainly regarded him with hostility. He favoured the moon god, Sin, rebuilding his temple in Haran. He was also an antiquarian. However, he may in fact have suffered from ulcers as tradition suggests (The prayer of Nabonidus from Qumran), and that would help to explain his retirement, and the remainder may simply have been due to his ‘scholarly' nature and dislike of functions, which would have been interpreted as ‘odd', if not worse.
Thus his son ruled for many years in Babylon as a junior co-regent, with the powers, if not the name, of kingship. The title of ‘sharru' (overall king) was never applied to him and he was rather entitled officially ‘mar sharri' (son of the overall king). But the title melek (king) was regularly applied to under-kings, and Belshazzar could thus be called ‘melek of Babylon'.
Nabonidus returned to his duties in the last part of his reign and just prior to this incident, was defeated by the forces of Cyrus at Sippar, and fled. At this time Belshazzar was still ruling in Babylon at the time this chapter commences. (Nabonidus later returned to Babylon and was captured).