Matthew Poole and continuators on Ezra 6:22

22 And kept the feast of unleavened bread seven days with joy: for the LORD had made them joyful, and turned the heart of the king of Assyria unto them, to strengthen their hands in the work of the house of God, the God of Israel.

i. e. Of the King of Persia, which was now king of Assyria also, or Emperour of that vast and famous Assyrian Empire: which was first subdued by the King of Babylon, who therefore is somwhere called the Assyrian; and for the same reason the Persian Monarch is here so called emphatically, to note the great Power and Goodness of God in turning the hearts of these great Monarchs, whose predecessors had been the chief persecutors and oppressors of Gods People.