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.