In order to properly understand the spiritual meaning of certain Scriptures it is necessary to understand exactly who the king of the Medes and the Persians represents. God helps us in our understanding through these two statement found in the book of Isaiah.
Isaiah 44:28 That saith of Cyrus, He is my shepherd, and shall perform all my pleasure: even saying to Jerusalem, Thou shalt be built; and to the temple, Thy foundation shall be laid.
Isaiah 45:1 Thus saith the LORD to his anointed, to Cyrus, whose right hand I have holden, to subdue nations before him;
Cyrus, the king of the Medes and Persians, is said to be God’s shepherd. Of course this ties in with what is said concerning the Lord Jesus:
John 10:11 I am the good shepherd: the good shepherd giveth his life for the sheep.
The Lord says Cyrus is His shepherd. And the Bible tells us Jesus is the Good Shepherd.
Also, the Lord declares that Cyrus is His anointed. The Hebrew word (#4899) translated as ‘anointed’ is the same word translated as “Messiah” in this verse:
Daniel 9:25 Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: ...
26 And after threescore and two weeks shall Messiah be cut off,
One further detail about Cyrus we should know, and that is that he is also known as Darius.
We know that the king of Babylon is a type of Satan and the kingdom of Babylon is a figure of Satan’s kingdom of this world. And now we know that Cyrus/Darius is a clear type of Christ, God’s Messiah.
Therefore we are now able to understand the spiritual meaning of this historical parable:
Daniel 5:30 In that night was Belshazzar the king of the Chaldeans slain.
31 And Darius the Median took the kingdom,...
The kingdom of Babylon (world) and the king of Babylon (Satan) were caught off guard as Cyrus/Darius came like a thief in the night and took the kingdom of Babylon and slew its king.
This fits in with what is said in these verses:
1 Thessalonians 5:2 For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.
3 For when they shall say, Peace and safety; then sudden destruction cometh upon them, as travail upon a woman with child; and they shall not escape.