High Quality Content by WIKIPEDIA articles! The Battle of Opis, fought in September 539 BC, was a major engagement between the armies of Persia under Cyrus the Great and the Neo-Babylonian Empire under Nabonidus during the Persian invasion of Mesopotamia. At the time, Babylonia was the last major power in western Asia that was not yet under Persian control. The battle was fought in or near the strategic riverside city of Opis, north of the capital Babylon. It resulted in a decisive defeat for the Babylonians. A few days later, the city of Sippar surrendered to the Persians and Cyrus`s forces entered Babylon apparently without a fight. Cyrus was subsequently proclaimed king of Babylonia and its subject territories, thus ending the independence of Babylon and incorporating the Babylonian Empire into the greater Persian Empire.