Ahasuerus, meaning the Mighty or Venerable king, was a title of certain Persian kings. Some think the one here was Xerxes, the son of Darius Hystapis, but this could not be , for the events of the book had to take place much earlier than this. It is clear from v2:5-7 that Mordecai was the cousin of Esther. She was his uncle's daughter, and he had brought her up because she had no father or mother. Mordecai had been taken captive with Jehoiachin a.k.a Coniah/Jeconiah (1 Chronicles 36:8-9; Jeremiah 52:31) , king of Judah, by Nebuchadnezzar. This was 11 years before the final captivity of Judah and destruction of Jerusalem (2 Kings 24:1-25,30; Jeremiah 25:1). The servitude to Babylon for 70 years began in the 4th years of Jehoikim (Jeremiah 25:1, 9:11)
If Mordecai was a young man, say 20 years old at the time of his captivity 11 years after the beginning of the 70 years servitude - about 627 B.C.E. - and Xerxes reigned 485-464 B.C.E., this would make Mordechai 162 years old in the first year of Xerxes. Esther would be an old woman by this time also.
The fact is that Esther was the young and beautiful wife of Darius the Mede, of Daniel chapter 5 and the mother of Cyrus, the commander of the Persian armies which took Babylon at the end of the 70 years of servitude. This Dairus (the father of Cyrus) reigned 35 years, 33 of which were during the later part of the 70 years of servitude of Israel to Babylon. This fits into all the historical facts about Mordecai being carried away captive with Jehoiachin as very young man. It is possible that he lived throughout the entire captivity. He could have easily been the Mordecai of Ezra 2:2; Nehemiah 7:7. Some Jews did live through the 70 years of servitude and returned to see the foundation of the temple laid (Ezra 3:12)
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Daniel 11:2
1. Cyrus reigned 2 years after Astyages or Darius the Mede had taken Babylon in 538 B.C.E. and resigned 9 years.
2. Cambyses, the son of Cyrus, reigned 7 years over Persia, 527-520 B.C.E.
3. Darius-1 reigned 35 years, 520-485 B.C.E. He was the first king of Persia to invade Greece, but was defeated at the battle of Marathon 490 B.C.E.
4. Xerxes reigned 21 years, 485-464 B.C.E. He was the Persian king that stirred up all against the realm of Grecia fulfulling Daniel 11:2. He also was defeated by the Greeks in 480-479 B.C.E.
Kings of Persia After Xerxes:
1. Artaxerxes-1 surnamed "The long-armed" reigned 41 years, 464-424 B.C.E.
2. Xerxes-1 reigned only 1 year 423 B.C.E.
3. Darius-2 reigned 19 years, 423-404 B.C.E.
4. Artaxerxes-2 reigned 46 years, 404-358 B.C.E.
5. Artaxerxes-3 reigned 20 years, 358-338 B.C.E.
6. Darius-3 reigned 8 years 338-330 B.C.E.
Darius-3 is the king that was defeated by Alexander the Great whose empire succeeded the Medo-Persian empire.