Albert Barnes Commentary Ezra 6:22

"and kept the feast of unleavened bread seven days with joy: for Jehovah had made them joyful, and had 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." — Ezra 6:22 (ASV)

The king of Assyria - that is, Darius. Assyria had so long been the great monarchy of western Asia that the sacred writers continue the title to those who had inherited the old Assyrian power, as first to the Babylonians (2 Kings 23:29), and secondly to the Persians. With similar inexactness, we find Herodotus calling Cyrus “king of the Medes.”