Assyria was a major ancient Mesopotamian civilization that existed as a city-state from the 21st century BC to the 14th century BC and eventually expanded into an empire from the 14th century BC to the 7th century BC. Assyrian history spans from the early Bronze Age to the late Iron Age; modern historians typically divide ancient Assyrian history into the Early Assyrian, Old Assyrian, Middle Assyrian, Neo-Assyrian, and post-imperial periods, based on political events and gradual changes in language. Assur, the first Assyrian capital, was founded c. 2600 BC, but there is no evidence that the city was independent until the collapse of the Third Dynasty of Ur, in the 21st century BC, when a line of independent kings starting with Puzur-Ashur I began ruling the city. Centered in the Assyrian heartland in northern Mesopotamia, Assyrian power fluctuated over time. The city underwent several periods of foreign rule or domination before Assyria rose under Ashur-uballit I in the early 14th century BC as the Middle Assyrian Empire.