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.