The Cyrus Cylinder is an ancient clay cylinder, now broken into several pieces, on which is written a declaration in Akkadian cuneiform script in the name of Persia's Achaemenid king Cyrus the Great. It dates from the 6th century BC and was discovered in the ruins of Babylon in Mesopotamia (modern Iraq) in 1879. It is currently in the possession of the British Museum, which sponsored the expedition that discovered the cylinder. It was created and used as a foundation deposit following the Persian conquest of Babylon in 539 BC when the Neo-Babylonian Empire was invaded by Cyrus and incorporated into his Persian Empire.

The text on the Cylinder praises Cyrus sets out his genealogy and portrays him as a king from a line of kings. The Babylonian king Nabonidus, who was defeated and deposed by Cyrus, is denounced as an impious oppressor of the people of Babylonia and his low-born origins are implicitly contrasted to Cyrus' kingly heritage. The victorious Cyrus is portrayed as having been chosen by the chief Babylonian god Marduk to restore peace and order to the Babylonians. The text states that Cyrus was welcomed by the people of Babylon as their new ruler and entered the city in peace. It appeals to Marduk to protect and help Cyrus and his son Cambyses.

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