Divine Kingship & the Mandate to Rule
Kingship was lowered from heaven — a sacred office on loan, withdrawn the moment the gods chose another.
Also: divine kingship · nam-lugal · kingship lowered from heaven · sacred kingship · deified king · mandate of the gods
In Mesopotamian thought, kingship was not invented by people but 'lowered from heaven' by the gods — a divine office bestowed on one man and, through him, on his city. The king ruled as the gods' chosen steward: he was called, named, and equipped by them, and his duty was to maintain justice, build and feed the temples, and defend the land. When a dynasty fell, it was because the gods had withdrawn kingship and granted it elsewhere. Some kings (notably in the Ur III period) were even deified, but kingship as such was always understood as a sacred trust held on the gods' behalf.
In short
The ideology that legitimate kingship (Sumerian nam-lugal) is a divine institution 'lowered from heaven,' granted by the gods to a chosen man and to a chosen city.
Where it’s found
'The Sumerian king list' (ETCSL 2.1.1, in corpus); the Ur III and Isin royal praise poems (Šulgi A–, ETCSL 2.4.2.*, in corpus)
How the idea developed across Sumer, Babylon & Assyria
Early Sumerian kings rule as stewards of the city-god; in the Akkadian and Ur III periods some kings (Naram-Sin, Šulgi and successors) are themselves deified; in Assyria and Babylonia the king is the gods' viceroy but not normally a god. The 'lowered from heaven' formula opens the Sumerian King List.
The King List's 'kingship lowered from heaven' framing is legitimacy ideology, not history.