Belshazzar played a pivotal role in the coup d´état that overthrew the king Labashi-Marduk (r. 556 BC) and brought Nabonidus to power in 556 BC. Since Belshazzar was the main beneficiary of the coup, through confiscating and inheriting Labashi-Marduk´s estates and wealth, it is likely that he was the chief orchestrator. Through proclaiming his father as the new king, Belshazzar also made himself the first-in-line to the throne. As Nabonidus was relatively old at the time, Belshazzar could expect to become king within a few years.
Nabonidus was absent from Babylon from 553 BC to 543 or 542 BC, in self-imposed "exile" at Tayma in Arabia, for unknown reasons. For the duration of the decade-long absence of his father, Belshazzar served as regent in Babylon, a period which some historians characterize as a co-regency. Belshazzar was entrusted with many typically royal prerogatives, such as granting privileges, commanding portions of the army, and receiving offerings and oaths, though he continued to be styled as the crown prince (mār šarri, literally meaning "son of the king"), never assuming the title of king (šarru). Belshazzar also lacked many of the prerogatives of kingship, most importantly he was not allowed to preside over and officiate the Babylonian New Year´s festival, which was the exclusive right of the king himself. Belshazzar´s fate is not known, but is often assumed that he was killed during Cyrus the Great´s Persian invasion of Babylonia in 539 BC, presumably at the fall of the capital Babylon on 12 October 539 BC.
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