The fall of Babylon was the decisive event that marked the total defeat of the Neo-Babylonian Empire to the Achaemenid Persian Empire in 539 BCE.
Nabonidus, the final Babylonian king and son of the Assyrian priestess Adad-guppi, ascended to the throne in 556 BCE, after overthrowing his predecessor Labashi-Marduk. For long periods, he would entrust rule to his son Belshazzar, who was a capable soldier but a poor politician. As a result, he was somewhat unpopular with many of his subjects, particularly the priesthood and the military class. To the east, the Persians had been growing in strength under the leadership of Cyrus the Great, who would soon lead a military expedition into Babylonian territory. In October 539 BCE, after the Battle of Opis, the Persian army made entry into the capital city of Babylon. With the success of Cyrus' campaign, Babylonia was incorporated into the Persian realm as a satrapy. As recorded in the Cyrus Cylinder, Cyrus vowed to respect the Babylonian people and also allowed incarcerated peoples to return to their homelands, most notably including the Babylonians' captives from Judah. Consequently, he was viewed as the legitimate successor of the ancient Babylonian kings, and, unlike Nabonidus, rose to become a popular figure in Babylon itself.
Conditions
A number of factors arose which would ultimately lead to the fall of Babylon. The population of Babylonia became restive and increasingly disaffected under Nabonidus. The Marduk priesthood hated Nabonidus because of his suppression of Marduk's cult and his elevation of the cult of the moon-god Sin. He excited a strong feeling against himself by attempting to centralize the religion of Babylonia in the temple of Marduk at Babylon, and thus alienated the local priesthoods.
The military party also despised him on account of his antiquarian tastes. He seemed to have left the defense of his kingdom to Belshazzar (a capable soldier but poor diplomat who alienated the political elite), while occupying himself with the more congenial work, like excavating foundation records of the temples and determining the dates of their builders. He also spent time outside Babylonia, rebuilding temples in the Assyrian city of Harran, and also among his Arab subjects in the deserts to the south of Mesopotamia.
Nabonidus and Belshazzar's Assyrian heritage is also likely to have added to this resentment. In addition, Mesopotamian military might had usually been concentrated in the martial state of Assyria. Babylonia had always been more vulnerable to conquest and invasion than its northern neighbour, and without the might of Assyria to keep foreign powers in check, Babylonia was ultimately exposed.
Preparations
It was in the sixth year of Nabonidus (550/549 BC) that Cyrus the Great, the Achaemenid Persian king of Anshan in Elam, revolted against his suzerain Astyages, king of the Manda or Medes, at Ecbatana. Astyages' army betrayed him to his enemy, and Cyrus established himself at Ecbatana, thus putting an end to the massive Median Empire and making the Persian faction dominant among the Iranic peoples.
Three years later Cyrus had become king of all Persia, and was engaged in a campaign to put down a revolt among the Assyrians in 547 BCE. Meanwhile, Nabonidus had established a camp in the desert of his colony of Arabia, near the southern frontier of his kingdom, leaving his son Belshazzar (Belsharutsur) in command of the army. In 540 BCE, according to Dougherty and S.Smith, Cyrus invaded Syria, most of Babylon's eastern possessions. In a few months, many of Nabonidus’s vassals were under Persian authority. Nabonidus had to return to Babylon in 543 BCE due to Cyrus constantly raiding the border.
Invasion
In 539 BCE, Cyrus invaded Babylonia. Historical reconstruction of the fall of Babylon to Persia has been problematic, due to the inconsistencies between the various source documents. Both the Babylonian Chronicles and the Cyrus Cylinder describe Babylon being taken "without battle", whereas the Greek historians Herodotus and Xenophon report that the city was besieged. The biblical Book of Daniel notes that the king was killed.
According to Xenophon, Belshazzar was killed in this conflict, but his account is not widely accepted. Nabonidus surrendered and was deported. Gutian guards were placed at the gates of the great temple of Bel, where the services continued without interruption. Cyrus did not arrive until 28/29 October, with Gobryas having acted for him in his absence. Gobryas was then made governor of the province of Babylon.
Babylon, like Assyria, became a colony of Achaemenid Persia in 539 BCE.