Daniel 8:20 As for the ram that you saw with the two horns, these are the kings of Media and Persia.
Media or the Medes or Madai, is a son of Japheth. Madai is Noah’s grandson.
Genesis 10:2 The sons of Japheth: Gomer, Magog, Madai, Javan, Tubal, Meshech, and Tiras.
Historically, the Medes and Persians formed the Persian or the Achaemenid Empire.
Daniel was not written in chronological order. Historically, Daniel 8 was written before Daniel 5.
We know Daniel received understanding of the vision from Daniel 8:1 in the third year of the reign of King Belshazzar to be about 553 BC.
In receiving the vision, Daniel understood Babylon would no longer be a player on the world stage and the Babylonian Empire was on its last legs. Recall, Daniel was physically in Babylon. At the time of the vision, Babylon was the head of gold of world empires from Nebuchadnezzar’s dream in chapter 2.
In 553 BC, The Persians were led by Cyrus the Great (559 BC to 529 BC). At the time of Daniel’s vision, secular history records Cyrus was yet to invade and capture Babylon. Secular history records that would happen 14 years later in 539 BC. Legend has it, Babylon was taken by the Persians without any conflict.
This is where secular history and the Bible collide. Secular history says Cyrus captured Babylon. The Bible says Darius the Mede captured Babylon. Secular history has no mention of Darius the Mede. Critics of the Bible view the character of Darius the Mede as a work of fiction. Darius the Mede would continue to be a part of Daniel’s writings in chapters 6 and 9. Some critics even go to the extent Daniel never existed.
King Belshazzar’s reign would come to an end in Daniel 5 with the hand writing on the wall.
Daniel 5:30-31 That very night Belshazzar the Chaldean king was killed. And Darius the Mede received the kingdom at the age of sixty-two.
The Persian Empire was on the cusp of its greatest power. Darius I would rule from 522 BC to 486 BC. Under his leadership, this would be the largest empire known to man at that time. It was often referred to as “The Empire”. Historical dates provided by Wikipedia.
https://en.wikipedia.org/wiki/Achaemenid_Empire
The eastern border of The Empire was essentially the Indus River which runs through modern day Pakistan. The southern border in the eastern part of the empire was the 21st century Persian Gulf. The southern border extended west across what is northern Saudi Arabia into what is modern day Egypt and eastern Libya. The northern border ranged from what was the old Soviet republics in the east to the Balkans to modern day Turkey moving west. It is in the far northwest corner of the Achaemenid Empire where resistance was ultimately encountered in Macedonia or what we know as modern day Greece.
The Empire would come to an abrupt end as Gabriel is about to inform Daniel.
