What it was. Persia, the Achaemenid empire, is the last of the four powers, and the one that reversed the exile. Cyrus the Great united the Medes and Persians, defeated Lydia and then Babylon, and built an empire that stretched, the ISBE says, "from the Indus to the Black Sea, from the Jaxartes to beyond the Nile" (ISBE, "Persians," III). Its rulers governed through satraps and were, by the standards of the age, tolerant of subject peoples' religion. The Bible knows Persia only from the exilic books onward (Ezra, Nehemiah, Esther, Daniel, and the late prophets); before that, the ISBE notes, the Persians were reckoned among the Medes.
How it dealt with the Jews. Persia is the empire of the return. Cyrus reversed Babylon's deportation policy, sending captive peoples and their gods back to their own lands, and for Judah this meant the decree to go home and rebuild the temple (2 Chronicles 36:22–23; Ezra 1:1–4). The work, stalled and restarted, was finished under Darius I (Ezra 6; Haggai; Zechariah); Ezra and Nehemiah came up later under Artaxerxes I to restore the law and the walls; and the book of Esther is set at the court of Ahasuerus (Xerxes). The line of Persian kings the Bible names runs until Alexander of Macedon ended the empire in 331 BC. (Source: ISBE 1915, "Persia"; "Persians," III.)