Assyria was the northern Mesopotamian kingdom centred on the cities of Ashur, Nimrud and Nineveh, and its distinctive religious feature is the near identity of its chief god with the political entity itself. Ashur is the name of the city, of the god and of the country, and the god has almost no mythology of his own: what he has is a temple, a king who serves as his priest, and an obligation on that king to extend his territory.
That produced a body of royal inscription in which conquest is reported as an act of worship. Campaigns are undertaken at the god's command and at his direction, defeated cities are described as punished for breaking oaths sworn by him, and the booty and tribute are brought into his house. The annals are propaganda, and their patterned exaggeration is well understood, but they are also the fullest surviving account anywhere of how an ancient state understood war as a religious duty.
Assyria also assembled and preserved. The library collected at Nineveh under Ashurbanipal in the seventh century BCE gathered omen series, incantations, rituals, hymns and literature from across Mesopotamia, including the fullest text of the Gilgamesh epic, and it survived because the city was burned in 612 BCE and the clay was baked rather than destroyed. Much of what is known about Mesopotamian religion in general comes from an Assyrian king's collecting habit.