Medo-Babylonian conquest of the Assyrian Empire in the context of "Nebuchadnezzar II"

The post-imperial period was the final stage of ancient Assyrian history, covering the history of the Assyrian heartland from the fall of the Neo-Assyrian Empire in 609 BC to the final sack and destruction of Assur, Assyria's ancient religious capital, by the Sasanian Empire c. AD 240. There was no independent Assyrian state during this time, with Assur and other Assyrian cities instead falling under the control of the successive Median (615–549 BC), Neo-Babylonian (612–539 BC), Achaemenid (539–330 BC), Seleucid (312–c. 141 BC) and Parthian (c. 141 BC–AD 224) empires. The period was marked by the continuance of ancient Assyrian culture, traditions and religion, despite the lack of an Assyrian kingdom. The ancient Assyrian dialect of the Akkadian language went extinct however, completely replaced by Aramaic by the 5th century BC.

During the fall of Assyria in the Medo-Babylonian conquest of the Assyrian Empire 626–609 BC, northern Mesopotamia was extensively sacked and destroyed by Median and Babylonian forces. The Babylonian kings, who annexed large parts of Assyria cared little for economically or socially developing the region and as such there was a dramatic decline in population density. Many of the greatest cities of the Neo-Assyrian period, such as Nineveh, were deserted and others, such as Assur, decreased dramatically in size and population. The region only began the process of recovery under the rule of the Achaemenid Empire. After his conquest of Babylon in 539, the Achaemenid king Cyrus the Great returned the cult statue of the Assyrian national deity Ashur to Assur. The Achaemenid practice of not interfering with local cultures, and the organization of the Assyrian lands into a single province, Athura, allowed Assyrian culture to endure.

Assyrian continuity is the study of continuity between the modern Assyrian people, a recognised Semitic indigenous ethnic, religious, and linguistic minority in Western Asia (particularly in Iraq, northeast Syria, southeast Turkey, northwest Iran and in the Assyrian diaspora) and the people of Ancient Mesopotamia in general and ancient Assyria in particular. Assyrian continuity and Ancient Mesopotamian heritage is a key part of the identity of the modern Assyrian people. No archaeological, genetic, linguistic, anthropological, or written historical evidence exists of the original Assyrian and Mesopotamian population being exterminated, removed, bred out, or replaced in the aftermath of the fall of the Assyrian Empire. Modern contemporary scholarship "almost unilaterally" supports Assyrian continuity, recognizing the modern Assyrians (and Mandaeans) as the ethnic, historical, and genetic descendants of the East Assyrian-speaking population of Bronze Age and Iron Age Assyria specifically, and (alongside the Mandeans) of Mesopotamia in general, which were composed of both the old native Assyrian population and of neighboring settlers in the Assyrian heartland.

Due to an initial long-standing shortage of historical sources beyond the Bible and a handful of inaccurate and contradictory works by a few later classical European authors, many "Western" historians prior to the early 19th century believed Assyrians (and Babylonians) to have been completely annihilated, although this was never the view in the region of Mesopotamia itself or surrounding regions in West Asia, where the name of the land continued to be applied until the mid 7th century AD, and Assyrian people have continued to be referenced as such through to the present day.