The Middle Assyrian king Assur-uballit is usually remembered as the first in a series of strong militaristic rulers who expanded the small state around Assur to the size of an empire demanding the acknowledgment of the other great powers of the time - Kassite Babylonia, the Hittites, and Egypt.
The first major wave of expansion occurred in the context of the political instability rocking the neighboring Mitanni state, which before Assur-uballit's time had been a major kingdom in competition with the Hittites and Egypt. After a remarkable phase of power and prominence during the reign of Saushtatar (c. 1430), a devastating invasion by the Hittite king Suppiluliuma (c. 1370-1330) destabilized the Mitanni royal family, resulting in the assassination of king Tushratta (Kuhrt pg. 253) and the establishment of Shattiwaza (one of Tushratta's sons) as vassal-king under the Hittites. The state underwent further fragmentation with the secession of the eastern region under one Artatama II (Kuhrt pg. 295).
Against this background Assur-uballit made his first power move, overpowering Artatama's fledgling state and emerging as a growing threat to the rest of the Mitanni state. This conquest substantially enlarged the Assyrian state and put it on the same political level as the Hittes, Egyptians, and Babylonians. As an inauguration of this new status, Assur-uballit sent two diplomatic letters to Amenophis IV. The first, demure in tone, is merely a salutation accompanying a number of luxury gifts for the Pharoah (see Kuhrt p. 350). However the second, written after the fall of Mitanni, presents the Assyrian king as a 'brother' of the Pharoah and therefore worthy of royal gifts equal to those previously given to the Mitanni (Kuhrt. pg. 351).
Despite Assyria's rise to status as a major kingdom, the historically dominant power of the south, Babylonia, at first refused to recognize its northern neighbor's position. Upon learning of the Assyrian delegation at the Egyptian court, the Kassite king Burnaburiash II (c. 1359 - 1333) sent of a brief letter to the Pharoah claiming that the Assyrians were still his vassals, and asking that their unauthorized mission be ignored. (See Kuhrt pg. 352).
Nevertheless during Assur-uballit's reign a royal marriage was effected between the two Mesopotamian states, with Burnaburiash's son Karaindash marrying the Assyrian king's daughter Muballitat-Sherua. The heir from this union would presumably rule in both parties' interests. When the new king Karahardash ascended the throne however, he was murdered by 'Kassite troops' (Kuhrt pg. 352) and replaced by a man of their own choice. This resulted in a second invasion by Assur-uballit and elevation of another member of the Babylonian royal family - Kurigalzu II, to the throne. This king ruled Babylonia for many years, albeit in frequent conflict with Assyria.