Battle of Opis

Cyrus’s Army Possibly learning from the Lydians, Cyrus created what some military historians consider the first true cavalry, fielding units of mounted warriors not as supplements to chariots but as their own force. It would not be long before chariots disappeared from the battlefield altogether (except in Britain, where they lasted another 750 years). Cyrus encouraged military innovation: during his invasion of Babylonia, his engineers managed to divert the course of the entire Euphrates, and he created a system of roads that served both armies and merchants well. His personal bodyguard, said to number 10,000 men, were called the “Immortals” because as soon as one died another would take his place, creating the impression of invincibility both within and outside the unit.

The Battle of Opis, in 539 BC, between the armies of Persia under Cyrus the Great and the Neo-Babylonian Empire under Nabonidus during the Persian invasion of Mesopotamia.The battle was fought near the strategic riverside city of Opis, north of the capital Babylon. It resulted in a decisive defeat for the Babylonians. Cyrus the Great was subsequently proclaimed king of Babylonia and its subject territories, thus incorporating the Babylonian Empire into the greater Persian Empire.

While campaigning against Lydia, Cyrus had made peaceful overtures to the Neo-Babylonian Empire ruled by Nabonidus, a Chaldean and successor to the great King Nebuchadnezzar II. However, in 539 bc, Cyrus invaded. Nabonidus met him on the field at Opis, a city probably on the Tigris where Nebuchadnezzar had built a massive dam as part of Babylon’s already impressive defenses. No details of the Battle of Opis survive, but the Babylonians suffered a devastating defeat; one of the casualties was Nabonidus’s own son. After that, Babylonia fell easily into Cyrus’s hands, in part because Nabonidus was universally disliked by his subjects. In particular, the Jews, who had been forced into exile in Babylon by Nebuchadnezzar, welcomed Cyrus as their deliverer.

The conquest of Babylon

When Babylonian King Nabonidus ascended the throne of Babylon in 556 BC, his kingdom had been allied to the Iranians for nearly 75 years. However, Cyrus’ conquest of Lydia changed the strategic balance between the Iranians and Babylon dramatically, and Babylon was invaded by Cyrus in 539 BC.

A major factor facilitating Cyrus’ conquest of Babylon was the unpopularity of Nabonidus amongst his own people, especially the priesthood. Nabonidus’ interest in the northern Mesopotamian moon god Sen at Harran, and his neglect of sacred duties necessary for the Babylonian god Marduk had alienated the priesthood. Nabonidus departed for the deserts of northwest Arabia in 540 BC, where he took up residence in the oasis town of Taima. When Cyrus invaded Babylon, he found a population unwilling to support their king. Cyrus’ diplomacy also won Gubaru (Ugbaru), the disaffected Babylonian governor of Gutium, over to the Achaemenids. Gubaru was a formidable general who had served the late Babylonian king Nebuchadnezzar (605-562 BC). His military support was to prove decisive in Cyrus’ conquest of Babylon.

Mesopotamian linear barriers

The “Wall of Babylon” was, in fact, not the walled city of Babylon but the “Median Wall” between the Tigris and Euphrates rivers, built by Nebuchadnezzar to block any potential Median thrust from Babylon’s northwest. The right end of the wall (at the Tigris) was supported by the fortress-city of Opis. The left end (at the Euphrates) was guarded by Sippar. The Tigris River, which guarded much of the eastern flank of the routes leading to the city of Babylon, was a difficult natural barrier against any invading army from the east. The strategic situation of the Median Wall in 539 BC bears some resemblance to the Maginot Line in 1940 in France. Both were built under the assumption that the enemy would invade along predictable axes of advance leading towards built-up fortifications. In neither case was any provision made for the possibility that the enemy would simply outflank the “wall” from another direction. Cyrus had no intention of predictably attacking across the Akkadian plains and expending himself against the Median Wall. His plan was to outflank that wall by way of a northern thrust. Thanks to earlier diplomacy, Cyrus’ troops would combine forces with the Babylonian contingents of Gubaru in a bid to strike at Opis and cross the Tigris, thereby outflanking the Median Wall to the southwest. Before striking Opis, Cyrus had to solve the problem of crossing the Tigris River to the rear of the fortress-city. Cyrus’ engineers are described by Herodotus as having worked for months to divert the water at the Gynades tributary of the Tigris into many separate channels.  

The draining of the Tigris allowed Cyrus to storm Opis in October. Few military details are available regarding the fighting; however, the forces that Cyrus defeated appear to have been a mix of Nabonidus’ regular army as well as Akkadian contingents. The capture of Opis and the crossing of the Tigris effectively outflanked the Median Wall. Once across the Tigris, Cyrus split his forces in two. He dispatched Gubaru’s troops alongside Persian contingents southwards towards Babylon City. Cyrus himself thrust southwest towards Sippar, which was also captured. The Babylonian army was now neutralized.

There appears to have been little popular resistance against Cyrus, which allowed the speedy advance of Cyrus’ forces into Babylon City. Nabonidus, who was now fleeing south, sought sanctuary in his capital, and was duly captured. The fall of Babylon City has been recorded as having been a peaceful and orderly affair, with Cyrus being welcomed as a liberator into the metropolis. This is corroborated by the Nabonidus Chronicle: “Cyrus entered Babylon … the state of peace was imposed on all the city, Cyrus sent greetings to all Babylon” It is certainly possible that pro-Cyrus Babylonian sympathizers may have helped Cyrus to secure the city. Cyrus entered the temple of Bel-Marduk and paid homage to the Babylonian god. The fate of Nabonidus is difficult to ascertain. One account asserts that Cyrus was magnanimous to his captive and allowed him to retire in comfort, allegedly exiling him to German (modern Kerman). A contradictory version is provided by Xenophon (431-350 Be) who reports Nabonidus being assassinated by Cyrus’ nobles in the great throne-room of Babylon. If true, this may be explained by Cyrus’ desire to placate the wishes of the Babylonian priesthood and populace.

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