Assyria was a major ancient Mesopotamian civilization of the Assyrians, which existed as a city-state controlling regional territories, then to a territorial state, and eventually an empire.
Assur, the first Assyrian capital, centered in the Assyrian heartland in northern Mesopotamia, Assyrian power fluctuated over time. The city underwent several periods of power together with several foreign rule or domination before Assyria rose under Ashur-uballit I, as the Middle Assyrian Empire.
In the Middle and Neo-Assyrian periods Assyria was one of the two major Mesopotamian kingdoms, alongside Babylonia in the south, and at times became the dominant power in the ancient Near East. Assyria was at its strongest in the Neo-Assyrian period, when the Assyrian army was the strongest military power in the world and the Assyrians ruled the largest empire then yet assembled in world history.
The Assyrian Empire fell later, conquered by a coalition of the Babylonians, Chaldeans, Medes, and Scythians, who had lived under the Assyrian rule for centuries. Though the core urban territory of Assyria was extensively devastated in the Medo-Babylonian conquest of the Assyrian Empire and the succeeding Neo-Babylonian Empire invested little resources in rebuilding it, ancient Assyrian culture and traditions continued to survive for centuries.
Assyria experienced a recovery under the Seleucid and Parthian empires, though declined again under the Sasanian Empire, which sacked numerous cities and semi-independent Assyrian territories in the region, including Assur itself.