The Conquests of Cyrus the Great

Cyrus the great was conqueror and founder of the Persian and Medes empire.  In 538 Cyrus, favored by a revolt of some Persian tribes, advanced on Babylon from the southeast, and, after giving battle to the army of Akkad, took Sippona and lastly Babylon without fighting. During a national feast in Babylon (the great empire at this time), his troops diverted the Euphrates river upstream, allowing his soldiers to enter the city through the lowered water.
The Persian army conquered the outlying areas of the city while the majority of Babylonians at the city center were unaware of the breach.   Cyrus thus became the kinf of Persia and Media, King of Babylon, and the great king of the four corners of the World.