What are common explanations for the angel being delayed for 21 days in Daniel 10:13? In the book of Daniel chapter 10, Daniel is in a period of mourning because a great war is coming. He fasts for 21 days and then sees a vision of what I presume to be an angel. Before the angel explains what will happen to Daniel's people, the angel explains why he is &quot;late&quot; in meeting him:

12 Then he said to me, &quot;Do not be afraid, Daniel, for from the first day that you set your heart on understanding this and on humbling yourself before your God, your words were heard, and I have come in response to your words.
13 &quot;But the prince of the kingdom of Persia was withstanding me for twenty-one days; then behold, Michael, one of the chief princes, came to help me, for I had been left there with the kings of Persia.
—Daniel 10:12–13, NIV

It seems very unusual to me that an angel would be delayed for three weeks by any force, and that Michael (the archangel?) had to come to help free him from the &quot;kings of Persia&quot; (demons?).
I suppose I've always had the image that an angel could just poof to a place instantaneously, as usually happens in popular media, so being stuck in a location for three weeks and needing a stronger angel to help seems really bizarre to me and doesn't seem to match up with depictions of other angels of the Bible (e.g., the ones that appeared to Mary and Joseph).
I'm really at a loss for how to explain this. What is an overview of the most common explanations for the angel being delayed here in Daniel?
The two most common explanations for the delay are:

To make an accommodation to the kingdom or guardian of Persia
To resist an evil intended by or suggested to the rulers of Persia

Accommodation to Persia
The view of an accommodation to Persia can be found primarily in church fathers like Jerome, Theodoret, and John Cassian.  This view typically understands the &quot;prince&quot; to be an archangel who has oversight over Persia (cf. Deuteronomy 32:8) and who resists the work of the angel out of a sense of justice for his own people.  Thus, Jerome writes:

And so the prince or angel of the Persians offered resistance, acting on behalf of the province entrusted to him, in order that the entire captive nation [Israel] might not be released.  [...] The prince of Persia opposed him for twenty-one days, enumerating the sins of the Jewish people as a ground for their justly being kept in captivity and as proof that they ought not to be released.1

This guardian, says Theodoret, was &quot;displeased&quot; that Israel would be blessed, despite their sins being worse than those of his own people, and thus the angel is delayed due to his debate with the Persian guardian over Israel's merit and God's plan for them.2
John Cassian's view of the prince is similar but less positive: he calls it a &quot;hostile power&quot; and says that it was out of &quot;jealousy&quot; that it delayed the angel.3
Resistance to an evil
The other common view, that the angel's delay was due to his resisting an evil, is held widely by Protestants in particular.  John Gill's explanation is typical:

Gabriel's business in the court of Persia was to work upon the minds of the king of Persia and his nobles, and to influence their counsels, and put them on such measures as would be in favour of the Jews, and be encouraging to them to go on in the rebuilding of their city and temple: in this he was withstood and opposed by an evil spirit that counterworked him; by exasperating the spirit of Cambyses against them.4

The Geneva Study Bible explains the angel's words by suggesting that Persia's rulers would have committed further evil against Israel:

Cambyses, who reigned in his father's absence, and did not only for this time hinder the building of the temple, but would have further raged, if God had not sent me to resist him: and therefore I have stayed for the profit of the Church.5

John Calvin,6 Matthew Poole,7 and John Wesley8 see the matter similarly.  Adam Clarke's interpretation is more gracious, seeing the &quot;prince&quot; as the fearful, not malicious, ruler of Persia:

Fearing, probably, the greatness of the work, and not being fully satisfied of his ability to execute it, [the king] therefore for a time resisted the secret inspirations which God had sent him.9

Who is the Prince?
The preferred interpretation of this verse has often depended on one's understanding of the &quot;prince&quot; of Persia: if a good angel, then the accommodation view, and if a bad angel or man, then the resistance view.  Albert Barnes notes that the language does not indicate whether the &quot;prince&quot; was good or bad, but because of his resistance to Daniel's angel, presumes him to be a bad angel.10  Haydock finds this argument wanting, saying that no angel, good or bad, can resist God's will.11

References:

Jerome, Commentary on Daniel, tr. Gleason L. Archer.
Theodoret, Commentary on Daniel, tr. Robert C. Hill, p 273–75
Conferences of John Cassian, 1.8.13
Gill, Exposition
Geneva Study Bible
Calvin, Commentary on Daniel
Poole, Annotations
Wesley, Notes
Clarke, Commentary
Barnes, Notes
Haydock, Catholic Bible Commentary