At which battle did Cyrus the Great defeat Babylonian forces in 539 BCE?

Cyrus the Great defeated Babylonian forces at the Battle of Opis in 539 BCE.

Opis was fought near the Tigris River during Cyrus’s campaign against the Neo-Babylonian Empire. Babylon was ruled by Nabonidus, whose government faced opposition in parts of the empire. Cyrus’s victory opened the way for Persian control of Babylonia.

Soon afterward, Persian forces entered Babylon, traditionally dated to October 539 BCE. The conquest greatly expanded Cyrus’s empire and gave him control of Mesopotamia, one of the ancient Near East’s richest regions.

The battle is sometimes confused with the later capture of Babylon itself or with Cyrus’s victory over Lydia at Thymbra. Opis was the battlefield victory; Babylon’s occupation followed it.