Aššur, God of the Assyrian Empire
God, city, and empire fused in one name — the king of Assyria ruled only as Aššur's viceroy.
Also: Ashur · Aššur · Assur · Assyria · national god · Nineveh
Assyria's national god, Ashur, began as the deity of the city that bore his name and rose, with the empire, to supremacy in the Assyrian north. Assyrian kings ruled explicitly as his viceroys, campaigned 'at the command of Ashur,' and reported their conquests to him. So central was he that Assyrian scribes re-edited the Babylonian creation epic to put Ashur in Marduk's place. The fusion of god, city, and empire was tightest here: to serve Assyria was to serve Ashur, and the king was first of all the god's appointed steward and high priest.
In short
Aššur, the city-god of the city Assur who became the national god of the Assyrian empire, in whose name its kings waged war and built, and to whom Enuma Elish was re-edited.
Where it’s found
Assyrian royal inscriptions (RINAP) and the Assyrian recension of Enūma Eliš (akkadian-pending)
How the idea developed across Sumer, Babylon & Assyria
Aššur is unusual: a deified city with little independent mythology who absorbs the functions of higher gods as Assyria expands (the Marduk-for-Aššur recension of Enuma Elish is the clearest case). Distinguish carefully: Aššur the god vs Assur the city vs Assyria the land.
Assyrian royal inscriptions are self-glorifying propaganda; quote their 'at the command of Aššur' claims as claims, not neutral report.