The Battle of Opis was the last major military engagement between the Achaemenid Persian Empire and the Neo-Babylonian Empire, which took place in September 539 BC, 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, located north of the capital city of Babylon in modern-day Iraq, and resulted in a decisive victory for Persia. Shortly afterwards, the Babylonian city of Sippar surrendered to Persian forces, who then supposedly entered Babylon without facing any further resistance. The Persian king Cyrus the Great was subsequently proclaimed as the king of Babylonia and its subject territories, thus ending its independence and incorporating the entirety of the fallen Neo-Babylonian Empire into the greater Achaemenid Empire.

By the year 540 BC, Cyrus captured Elam (Susiana) and its capital, Susa. The Nabonidus Chronicle records that, prior to the battle(s), Nabonidus had ordered cult statues from outlying Babylonian cities to be brought into the capital, suggesting that the conflict had begun possibly in the winter of 540 BC. Near the beginning of October 539 BC, Cyrus fought the Battle of Opis in or near the strategic riverside city of Opis on the Tigris, north of Babylon. The Babylonian army was routed, and on 10 October, Sippar was seized without a battle, with little to no resistance from the populace. It is probable that Cyrus engaged in negotiations with the Babylonian generals to obtain a compromise on their part and therefore avoid an armed confrontation. Nabonidus, who had retreated to Sippar following his defeat at Opis, fled to Borsippa.

Ancient Near East circa 540 BC, prior to the invasion of Babylon by Cyrus the Great Two days later, on 12 October (proleptic Gregorian calendar), Gubaru's troops entered Babylon, again without any resistance from the Babylonian armies, and detained Nabonidus. Herodotus explains that to accomplish this feat, the Persians, using a basin dug earlier by the Babylonian queen Nitokris to protect Babylon against Median attacks, diverted the Euphrates river into a canal so that the water level dropped "to the height of the middle of a man's thigh", which allowed the invading forces to march directly through the river bed to enter at night. Shortly thereafter, Nabonidus returned from Borsippa and surrendered to Cyrus. On 29 October, Cyrus himself entered the city of Babylon.

Prior to Cyrus's invasion of Babylon, the Neo-Babylonian Empire had conquered many kingdoms. In addition to Babylonia itself, Cyrus probably incorporated its subnational entities into his Empire, including Syria, Judea, and Arabia Petraea, although there is no direct evidence of this fact.

After taking Babylon, Cyrus the Great proclaimed himself "king of Babylon, king of Sumer and Akkad, king of the four corners of the world" in the famous Cyrus Cylinder, an inscription deposited in the foundations of the Esagila temple dedicated to the chief Babylonian god, Marduk. The text of the cylinder denounces Nabonidus as impious and portrays the victorious Cyrus pleasing the god Marduk. It describes how Cyrus had improved the lives of the citizens of Babylonia, repatriated displaced peoples, and restored temples and cult sanctuaries. Although some have asserted that the cylinder represents a form of human rights charter, historians generally portray it in the context of a long-standing Mesopotamian tradition of new rulers beginning their reigns with declarations of reforms.

Cyrus the Great's dominions composed the largest empire the world had ever seen to that point. At the end of Cyrus' rule, the Achaemenid Empire stretched from Asia Minor in the west to the Indus River in the east.

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