The king from the east was Cyrus, the Persian king who conquered Babylon in 539 BC. He allowed the Jews to return to their land and rebuild Jerusalem in 538 BC (Ezra 1:1–4).
Isaiah’s prophecies about Cyrus (Isaiah 44:28–45:13; 48:14–15) encouraged Israel to see the Lord as the ruler over all human history, including the actions of great kings.
He hands nations over to him: As victorious as the conquering Cyrus was, he was still subject to the Lord.