Battle of Opis The Battle of Opis , fought in September 539 BC , was a major engagement between the armies of Persia under Cyrus the Great and the Neo - Babylonian Empire under Nabonidus during the Persian invasion of Mesopotamia . At the time , Babylonia was the last major power in western Asia that was not yet under Persian control . The battle was fought in or near the strategic riverside city of Opis , north of the capital Babylon . It resulted in a decisive defeat for the Babylonians . A few days later , the city of Sippar surrendered to the Persians and Cyrus ' s forces entered Babylon apparently without a fight . Cyrus was subsequently proclaimed king of Babylonia and its subject territories , thus ending the independence of Babylon and incorporating the Babylonian Empire into the greater Persian Empire . Location . The site of the battle was at the city of Opis on the river Tigris , located about 50 miles ( 80 km ) north of modern Baghdad . The city is thought to have been a preferred point to cross the river ; Xenophon describes a bridge there . The timing of the invasion may have been determined by the ebb of the Mesopotamian rivers , which are at their lowest levels - and therefore are easiest to cross - in the early autumn . Opis was a place of considerable strategic importance ; apart from the river crossing , it was at one end of the Median Wall , a fortified defensive barrier north of Babylon that had been built several decades earlier by Nebuchadnezzar II . Control of Opis would have enabled Cyrus to break through the Median Wall and open the road to the capital . Sources . The main contemporary source of information on Cyrus ' s Mesopotamian campaign of 539 BC is the Nabonidus Chronicle , one of a series of clay tablets collectively known as the Babylonian Chronicles that record the history of ancient Babylonia . Some additional detail is provided by one of the few documents to have survived from Cyrus ' s lifetime , the Cyrus Cylinder . Further information on Cyrus ' s campaign is provided by the later ancient Greek writers Herodotus and Xenophon , though neither mention the battle at Opis and their accounts of the campaign differ considerably from the Persian and Babylonian sources . Most scholars prefer to use the Nabonidus Chronicle as the main source on the battle , as it is a contemporaneous source . Although much of the Nabonidus Chronicle is fragmentary , the section relating to the last year of Nabonidus ' s reign - 539 BC - is mostly intact . It provides very little information about Cyrus ' s activities in the years immediately preceding the battle . The chronicler focuses on events of immediate relevance to Babylonia and its rulers , only occasionally records events outside Babylonia and does not provide much detail other than a bare outline of key incidents . There is almost no information for the period 547 - 539 . Most of the chronicle ' s text for this period is illegible , making it impossible to assess the significance of the few words that can be read . Background . At the time of the Battle of Opis , Persia was the leading power in the Near East . Its power had grown enormously under its king , Cyrus II , who had conquered a huge swathe of territory to create an empire that covered an area corresponding to the modern countries of Turkey , Armenia , Azerbaijan , Iran , Kyrgyzstan and Afghanistan . The only remaining significant unconquered power in the Near East was the Neo - Babylonian Empire , which controlled Mesopotamia and subject kingdoms such as Syria , Judea , Phoenicia and parts of Arabia . It had been closely linked with Cyrus ' s enemies elsewhere . The empire was previously an ally of Croesus of Lydia , whose kingdom was overrun by the Persians a few years prior to the invasion of Babylonia . By the time of the battle , Babylonia was in an unpromising geopolitical situation ; the Persian empire bordered it to the north , east and west . It had also been suffering severe economic problems exacerbated by plague and famine , and its king Nabonidus was said to be unpopular among many of his subjects for his unconventional religious policies . According to Mary Joan Winn Leith , " Cyrus ' s success is credited to military acumen , to judicious bribery , and to an energetic publicity campaign waged throughout Babylonia , which portrayed him as a lenient and religiously tolerant overlord . " On the other hand , Max Mallowan notes : " Religious toleration was a remarkable feature of Persian rule and there is no question that Cyrus himself was a liberal - minded promoter of this humane and intelligent policy , " and such a publicity campaign was in effect a means of permitting his reputation to proceed his military campaign . Cyrus was said to have persuaded a Babylonian provincial governor named Gobryas ( and a supposed Gadates ) to defect to his side . Gutium , the territory governed by Gobryas , was a frontier region of considerable size and strategic importance , which Cyrus was said to have used as the starting point for his invasion . The Nabonidus Chronicle records that prior to the battle , Nabonidus had ordered cult statues from outlying Babylonian cities to be brought into the capital , suggesting that the conflict had begun possibly in the winter of 540 BC . In a fragmentary section of the chronicle which is presumed to cover 540 / 39 BC , there is a possible reference to fighting , a mention of Ishtar and Uruk , and a possible reference to Persia . The Battle of Opis was thus probably only the final stage in an ongoing series of clashes between the two empires . The battle . The Nabonidus Chronicle records that the battle took place in the month of Tashritu ( 27 September - 27 October ) " at Opis on the [ bank of the ] Tigris . " Very little is known about the events of the battle ; the chronicle does not provide any details of the battle ' s course , the disposition of the forces on either side or the casualties inflicted . The Persian army under Cyrus fought " the army of Akkad " ( meaning the Babylonians in general , not the city of that name ) . The identity of the Babylonian commander is not recorded in the chronicle but it has traditionally been assumed that Belshazzar , the son of Nabonidus , was in command . His fate is unclear and he may have been killed in the battle . The outcome of the battle was clearly a Babylonian defeat , possibly a rout , as the defeated Babylonian army is not mentioned again in the chronicle . Following the battle the Persian forces " took plunder " from the defeated Babylonians . Most translations of the Chronicle also refer to a " massacre " of " the people of Akkad " , though translators disagree on which side was responsible and who was killed - the population of Opis or the retreating Babylonian army . Pierre Briant comments : " This victory was followed by an immense haul of booty and the massacre of those who attempted to resist . " Andrew Robert Burn comments : " Indeed on one reading of the text , Akkad broke out into open revolt , and Nabonidus ' last military achievement was slaughter of rebels . " Maria Brosius interprets the massacre as a punitive action , " mak [ ing ] an example of a city trying to resist the Persian army . " Cuyler Young comments on the Chronicle accounts : " This reference in the Chronicle suggests that the Persians captured intact the main camp of Nabonidus ' army and that , as is so often the case , the real killing of the engagement came after the Babylonians had fallen prey to fear and panic and had retreated from the field . " Amélie Kuhrt comments that the references to a massacre and looting suggest that the battle was " probably a hard - won victory . " W . G . Lambert argues a contrarian view that there was no massacre or slaughter at all . The battle is not mentioned in the inscription on the Cyrus Cylinder , which portrays Cyrus as liberating Babylon peacefully and with the consent of its people . However , the battle demonstrates that the existing Babylonian regime actively resisted Cyrus ' s invasion of Mesopotamia . Aftermath . The defeat at Opis appears to have ended any serious resistance to the Persian invasion . The Nabonidus Chronicle states that following the battle , " on the fourteenth day [ 6 October ] Sippar was captured without battle . Nabonidus fled . " The chronicle ' s wording implies that Nabonidus was present in Sippar when the Persians arrived . Cyrus remained in Sippar , and " on the sixteenth day [ 12 October ] Ug / Gubaru , governor of Gutium , and the army of Cyrus without a battle entered Babylon . " Nabonidus himself was captured shortly afterward when he returned to Babylon . His ultimate fate is unclear , but according to the 3rd century BC Babylonian historian Berossus , Nabonidus was spared and he went into exile in Carmania , where he died years later . Persian troops took control of the city , though the Nabonidus Chronicle provides little detail of how this was done . The chronicle makes a point of noting the conquering army ' s protection of the city ' s most important temples and records that " Interruption of ( rites / cults ) in [ the ] Esagila [ temple ] or the [ other ] temples there was none , and no date was missed . " Seventeen days later , on 29 October , Cyrus himself entered Babylon , where he was proclaimed king , issued royal proclamations and appointed governors of his newly conquered realm . Ancient Greek accounts of Cyrus ' s campaign and the fall of Babylon differ significantly from the cuneiform accounts preserved in the Nabonidus Chronicle and the Cyrus Cylinder , suggesting that the Greeks were drawing on — or perhaps inventing — different traditions about the conquest of Babylonia . The two ancient Greek sources for the campaign , Herodotus and Xenophon , present broadly similar versions of events . According to Herodotus , Cyrus marched to Babylon along the side of the Diyala river ( past Opis , though the battle is not mentioned ) , where the Persians fought a battle with the Babylonians near the capital . Cyrus subsequently laid siege to Babylon , ordering his troops to dig a canal to drain off part of the Euphrates to enable his troops to penetrate the city through weak points in its defences . Xenophon provides a similar but more elaborate account , claiming that Cyrus dug a huge trench around the city to divert the Euphrates and make the river bed passable for the Persian army . Herodotus , Xenophon and the Biblical Book of Daniel all assert that the Babylonians were taken by surprise while celebrating a festival . Berossus presents an account that is different again , asserting that Cyrus defeated Nabonidus , who " fled with certain others and shut himself up in Borsippa . Meanwhile Cyrus occupied Babylon and ordered to destroy the exterior walls of the city , because the city seemed very formidable to him and difficult to capture . Afterward Cyrus marched to Borsippa , in order to organize the siege against Nabonidus . But Nabonidus did not await the end of the siege , and surrendered . " These accounts , written long after the Persian conquest , contradict many aspects of the contemporary cuneiform evidence , which does not mention any sieges , engineering works or battles near Babylon . The cuneiform descriptions of a peaceful surrender of Babylon are corroborated by archaeological evidence from the city , as no evidence of conflagrations or destruction have been found in the layers corresponding to the fall of the city to the Persians . Scholars are in general agreement that Herodotus ' s account is an invention , while Kuhrt comments that Xenophon ' s account in his " Cyropedia " is " virtually impossible to use . . . as a strictly historical source " due to its literary form , as a moral treatise on Cyrus in the form of an historical novella . Paul - Alain Beaulieu suggests that the Greek accounts may constitute an aggregate of various folk tales and legends which came to be associated with the fall of Babylon . " David George Hogarth and Samuel Rolles Driver comment on what they saw as Herodotus ' s unreliability : According to the Behistun Inscription , Babylon revolted twice against Darius , and was recaptured on the second occasion by his general Gobryas . Herodotus only mentions the first revolt of Babylon in which Zopyrus captured the city for Darius , and omits this second revolt . Historiography . The Babylonian defeat at Opis and the apparently unopposed Persian entry into Babylon ended the independence of Babylonia ( although there were a number of unsuccessful revolts against later Persian rulers ) . That the Babylonian collapse was swift and apparently total is confirmed by the ancient accounts of Cyrus ' s campaign in Mesopotamia and corroborating evidence such as cuneiform inscriptions dating to shortly after the Persian conquest . A number of explanations have been advanced for the rapid collapse of the Babylonian state . The Cyrus Cylinder and the roughly contemporary Verse Account of Nabonidus attribute Nabonidus ' s failure to the desire of the god Marduk to punish a regime that had opposed his will . The strongly anti - Nabonidus tone of these documents , which accused the former king of behaving capriciously and neglecting the worship of the gods , suggests that their authors - the Babylonian priestly elite - were alienated from Nabonidus and may have welcomed a Persian takeover . It is , however , unclear how widely the Persians were supported within Babylonia , as accounts of the invasion and Nabonidus ' s rule are coloured by Cyrus ' s subsequent propaganda . Other writers have advanced a number of additional or alternative explanations for the Babylonian defeat . M . A . Dandamaev suggests variously that the regime suffered from a lack of allies ; a lack of support among the general population ; opposition from subject peoples such as the Jews , who may have seen the invading Persians as liberators ; and the inability of the Babylonian forces to resist numerically superior and better equipped opponents .