The first inscriptions of Assyrian rulers appear after 2000 BC. Assyria then consisted of a number of city states and small Semitic kingdoms. The foundation of the Assyrian monarchy was traditionally ascribed to Zulilu, who is said to have lived after Bel-kap-kapu (Bel-kapkapi or Belkabi, ca. 1900 BC), the ancestor of Shalmaneser I.
City state of Ashur
The city-state of Ashur had extensive contact with cities on the Anatolian plateau. The Assyrians established "merchant colonies" in Cappadocia, e.g., at Kanesh (modern Kültepe) circa 1920 BC – 1840 BC and 1798 BC – 1740 BC. These colonies, called karum, the Akkadian word for 'port', were attached to Anatolian cities, but physically separate, and had special tax status. They must have arisen from a long tradition of trade between Ashur and the Anatolian cities, but no archaeological or written records show this. The trade consisted of metal (perhaps lead or tin; the terminology here is not entirely clear) and textiles from Assyria, that were traded for precious metals in Anatolia.
Like many commercial city-states in history, Assur was to a great extent an oligarchy rather than a monarchy. Authority was considered to lie with "the City", and the polity had three main centers of power — an assembly of elders, a hereditary ruler, and an eponym. The ruler presided over the assembly and carried out its decisions. He was not referred to with the usual Akkadian term for "king", šarrum; that was instead reserved for the city's patron deity Assur, of whom the ruler was the high priest. The ruler himself was only designated as "the steward of Assur" (iššiak Assur), where the term for steward is a borrowing from Sumerian ensi(k). The third centre of power was the eponym (limmum), who gave the year his name, similarly to the archons and consuls of Classical Antiquity. He was annually elected by lot and was responsible for the economic administration of the city, which included the power to detain people and confiscate property. The institution of the eponym as well as the formula iššiak Assur lingered on as ceremonial vestiges of this early system throughout the history of the Assyrian monarchy.
Kingdom of Shamshi-Adad I
The city of Ashur was conquered by Shamshi-Adad I (1813 BC – 1791 BC) in the expansion of Amorite tribes from the Khabur river delta. He put his son Ishme-Dagan on the throne of a nearby city, Ekallatum, and allowed the former Anatolian trade to continue. Shamshi-Adad I also conquered the kingdom of Mari on the Euphrates putting another of his sons, Yasmah-Adad on the throne there. Shamshi-Adad's kingdom now encompassed the whole of northern Mesopotamia. He himself resided in a new capital city founded in the Khabur valley, called Shubat-Enlil.
Ishme-Dagan inherited the kingdom, but Yasmah-Adad was overthrown, and Mari was lost. The new king of Mari allied himself with Hammurabi of Babylon. Assyria now faced the rising power of Babylon in the south. Ishme-Dagan responded by making an alliance with the enemies of Babylon, and the power struggle continued for decades.
Assyria reduced to vassal states
Hammurabi eventually prevailed over Ishme-Dagan, and conquered Ashur for Babylon. With Hammurabi, the various karum in Anatolia ceased trade activity — probably because the goods of Assyria were now being traded with the Babylonians' partners. Assyria was ruled by vassal kings dependent on the Babylonians for a century. After Babylon fell to the Kassites, the Hurrians dominated the northern region, including Assur.
There are dozens of Mesopotamian cuneiform texts from this period, with precise observations of solar and lunar eclipses, that have been used as 'anchors' in the various attempts to define the chronology of Babylonia and Assyria for the early second millennium, i.e., the "high", "middle", and "low" chronologies.