It may seem strange to discuss the identity of a Persian ruler in a series on the Greeks, but until the late nineteenth century, two Greek historians were our primary witnesses for the fall of Babylon to the Achaemenid (Medo-Persian) Empire in 539 B.C. History can be both enlightening and entertaining in its own right, but it is also instrumental in understanding the world the Bible was written in. Reading ancient historians therefore affords us additional insights to what the Bible says and what the Bible means, and is especially helpful concerning difficult passages of Scripture.
One particular problem they assist with is in determining the identity of the ruler known in the Bible as “Darius the Mede,” and Darius’ role in the Persian conquest of Babylon. In the fifth chapter of the book of Daniel, Daniel has just rebuked Belshazzar (the fifth king after Nebuchadnezzar) for using articles from the Jewish temple to worship “the gods of silver and gold, of bronze, iron, wood, and stone, which do not see or hear or know” (Dan 5:23 ESV), and so Daniel interprets to him the (literal!) handwriting on the wall:
“Then from his [God’s] presence the hand was sent, and this writing was inscribed. And this is the writing that was inscribed: MENE, MENE, TEKEL, and PARSIN. This is the interpretation of the matter: MENE, God has numbered the days of your kingdom and brought it to an end; TEKEL, you have been weighed in the balances and found wanting; PERES, your kingdom is divided and given to the Medes and Persians.”
The ESV Study Bible summarizes for us the historical problem posed by the passage:
The identity of Darius the Mede and the exact nature of his relationship to Cyrus is not certain. It is clear that Cyrus was already king of Persia at the time when Babylon fell to the Persians (539 B.C.), and thus far no reference to “Darius the Mede” has been found in the contemporary documents that have survived. (ESVSB on Dan 5:30-31)
Skeptics, of course, have been quick to jump on this unconfirmed claim to question the inspiration, authority, and inerrancy of Scripture. But judging by other historical sources from the period, the Bible appears to record these events in a very similar manner to other ancient sources. Another contemporary account that provides some additional insight is The Nabonidus Chronicle, which recounts many details from the reign of Belshazzar’s father, Nabonidus (more on this below):
In the month of Tashritu . . . . The 16th day, Gobryas (Ugbaru), the governor of Gutium and the army of Cyrus entered Babylon without battle. Afterwards Nabonidus was arrested in Babylon when he returned (there). Till the end of the month, the shield(-carrying) Gutians were staying within Esagila (but) nobody carried arms in Esagila and its (pertinent) buildings, the correct time (for a ceremony) was not missed. In the month of Arahshamnu, the 3rd day, Cyrus entered Babylon, green twigs were spread in front of him—the state of “Peace” (šulmu) was imposed upon the city. Cyrus sent greetings to all Babylon. Gobryas, his governor, installed (sub-)governors in Babylon. (ANET 306-7, Pritchard 281-282)
Less than one hundred years later, Herodotus records an expanded account of the conquest in book one of The Histories (190-191 in my Penguin edition):
The Babylonians had taken the field and were awaiting his [Cyrus’] approach. When he arrived near the city they attacked him, but were defeated and forced to retire inside their defences . . . and as they had taken the precaution of accumulating in Babylon a stock of provisions sufficient to last many years, they were able to regard the prospect of a siege with indifference. The siege dragged on, no progress was made, and Cyrus was beginning to despair of success. Then somebody suggested or he himself thought up the following plan: he stationed part of his force at the point where the Euphrates flows into the city and another contingent at the opposite end where it flows out, with orders to both to force an entrance along the riverbed as soon as they saw that the water was shallow enough. Then, taking with him all his non-combatant troops, he withdrew to the spot where Nitocris had excavated the lake, and proceeded to repeat the operation which the queen had previously performed: by means of a cutting he diverted the river into the lake (which was then a marsh) and in this way so greatly reduced the depth of water in the actual bed of the river that it became fordable, and the Persian army, which had been left at Babylon for the purpose, entered the river, now only deep enough to reach about the middle of a man’s thigh, and, making their way along it, got into the town. If the Babylonians had learnt what Cyrus was doing or had seen it for themselves in time, they could have let the Persians enter and then, by shutting all the gates which led to the waterside and manning the walls on either side of the river, they could have caught them in a trap and wiped them out. But as it was they were taken by surprise. The Babylonians themselves say that owing to the great size of the city the outskirts were captured without the people in the centre knowing anything about it; there was a festival going on, and they continued to dance and enjoy themselves, until they learned the news the hard way. That, then, is the story of the first capture of Babylon.
One final account is also worth mentioning, from Xenophon’s Cyropaedia. Xenophon was born around the time Herodotus died, and is thus even further removed from the events he described, but his account also supports some of the claims made by our earlier sources. We will pick up his account (which I have greatly abbreviated), with the decision to divert the river:
Thereupon Cyrus took his measurements all round the city, and, leaving a space on either bank of the river large enough for a lofty tower, he had a gigantic trench dug from end to end of the wall, his men heaping up the earth on their own side. . . . [When] the trenches were dug . . . Cyrus heard that it was a time of high festival in Babylon when the citizens drink and make merry the whole night long. As soon as the darkness fell, he set his men to work. The mouths of the trenches were opened, and during the night the water poured in, so that the river-bed formed a highway into the heart of the town.
You really should read the whole thing, which makes for great reading, if not good history (in fact, Ridley Scott could have learned a thing or two). But the parallels between these four accounts are unmistakable. The king of Babylon at this time is actually Nabonidus himself. He had usurped the throne seventeen years earlier, but had since retired into Arabia, leaving Belshazzar as regent of Babylon in his stead (perhaps explaining why the prince, as Number Two, could only make Daniel, “the third ruler in the kingdom”). Cyrus and his allies then approached from the north and northwest, respectively.
As is indicated in Herodotus and Nabonidus, the combat troops at Babylon appear to be under the command of Gobryas/Ugbaru, an old ally of Cyrus’, leading the breach of the city during the ceremonial festival hosted by Belshazzar. Cyrus himself arrived at the city several days later, incorporating Babylonia into the empire, and rewarding Ugbaru with the governorship of the region. Ugbaru then conducted further reforms, including the appointment of lieutenants throughout the city. The biggest differences are that Nabonidus makes no mention of the river works, Herodotus makes no mention of Ubgaru, and Xenophon makes Cyrus the primary actor throughout.
Which brings us back to our original question: who is “Darius the Mede,” and how does he fit into this picture? In the same paragraph cited above, The ESV Study Bible summarizes the three most common solutions proposed by biblical scholars:
. . . thus far no reference to “Darius the Mede” has been found in the contemporary documents that have survived.  That absence, however, does not prove that the references to Darius in the book of Daniel are a historical anachronism. The book of Daniel recognizes that Cyrus reigned shortly after the fall of Babylon (1:1; 6:28), and knowledge of the history of this period, while substantial, may be incomplete. Until fairly recently there was no cuneiform evidence to prove the existence of Belshazzar either.  Some commentators argue that Darius was a Babylonian throne name adopted by Cyrus himself. On this view, 6:28 should be understood as, “during the reign of Darius the Mede, that is, the reign of Cyrus the Persian.”  Others suggest that Darius was actually Cyrus’s general, elsewhere named Gubaru or Ugbaru, and credited in the Nabonidus Chronicle with the capture of Babylon. (ESVSB on Dan 5:30-31)
The first response is a general truth that applies to any aspect of biblical history: history cannot prove a biblical statement to be false due to a lack of evidence, and Christians should not withhold their assent to a biblical statement simply because of a lack of historical confirmation. But this should also not be seen as an excuse for not engaging with known historical sources, or to simply win arguments or silence debate (all of which applies equally well to contemporary discussions on faith and science). The second theory is certainly possible in a strictly grammatical way, but this construction seems forced, and receives little attention from Bible translators.
The most likely explanation of our known sources, then, is that Darius is the name chosen by the biblical writer to refer to the Gobryas/Ugbaru/Gubaru mentioned in our sources. This fits especially well with the wording seen in Daniel: Darius “received the kingdom” and “was made king over the realm of the Chaldeans,” implying that his authority was derived from a superior (such as Cyrus; Dan 5:30-31; 9:1, emphases added; see 6:28). There are also parallels between the reforms attributed to Darius/Ugbaru, such as the appointment of satraps/sub-governors throughout the realm. So while historians now doubt aspects of the historical accounts (most notably, the diversion of the river; see below, Note 1), these accounts also provide important contextual information supporting the biblical account in the book of Daniel.
On the other hand, we must also point out that the Bible does not necessarily require us to believe that Darius the Mede and Ugbaru are the same person; this is simply the best inference we can make given our current understanding of the biblical text and the history surrounding it. So while we may not be able to state absolutely who Darius is, we can certainly assent with his testimony, as recorded in Scripture:
Peace be multiplied to you. I make a decree, that in all my royal dominion people are to tremble and fear before the God of Daniel, for he is the living God, enduring forever; his kingdom shall never be destroyed, and his dominion shall be to the end. He delivers and rescues; he works signs and wonders in heaven and on earth, he who has saved Daniel from the power of the lions. (Dan 6:25-27)
Note 1: Such as Marincola in note 80 to The Histories. In the same place he also states, “it is clear that there was treachery, perhaps because of the discontent aroused by Nabonidus, particularly towards the priestly caste who may thus have welcomed Cyrus. It is likely too that the large Jewish community at Babylon assisted Cyrus’ entry into the city; after its fall Cyrus returned the Jewish exiles to Jerusalem, with orders for the temple there to be rebuilt (Ezra 6.3-22).” This, however, appears to be mere supposition.