The earliest record of Nebuchadnezzar III is a document mentioning him as the king of Babylon on 3 October 522 BC, possibly the day of his accession to the throne.

Nebuchadnezzar III quickly established his rule in Babylonia, seizing control of not only Babylon itself but also the cities of Borsippa, Sippar and Uruk.

The city, owing to its prestigious and ancient history, continued to be an important site, however, with a large population, defensible walls and a functioning local cult for centuries.

[9] Though the city did become one of the Achaemenid Empire's capitals (alongside Pasargadae, Ecbatana and Susa), retaining some importance through not being relegated to just a provincial city,[10] the Persian conquest introduced a ruling class which was not absorbed by the native Babylonian culture, instead maintaining their own additional political centers outside of Mesopotamia.

[17] Nebuchadnezzar sought to make himself the ruler of an autonomous Babylonia[2] and his revolt had probably originally been aimed at throwing off the rule of the unpopular Bardiya.

[18] Although Darius was quick to move against Nebuchadnezzar,[19] his rule lasted uninterrupted for a few months, at least in Babylon itself and in the cities of Borsippa,[20] Sippar[18] and Uruk.

[21] In early December, Darius's army was nearing the Tigris river, intending to cross it to enter Babylonia.

In order to halt the Persian advance, Nebuchadnezzar stationed his troops in the reed thickets of the river, hoping to guard the crossing and seize Darius's boats.

[2] On 18 December, the Babylonians suffered a decisive defeat in a second battle, which took place near Zazana by the Euphrates river.