Near the end of 13th century BC, Assyria endured a period of short kingly reigns. We approach the aftermath of Babylonia earning her independence from Assyrian occupation, however, with these two states once in bed together, each held the other in awkward standing, with Assyria spurned, and with Babylonia whose Kassites' affections had since run cold.
1. King Ashur-nadin-apli is succeeded by Ashur-nirari III (1213-1208).
Like his predecessor, we know little of Ashur-nirari III's reign, with one exception: An interaction between he and the Kassites via letters styled similarly to the characters of Hammurabi's day which were icy in tone, alluding to growing hostilities between Assyria and Babylonia since the grand schism of the two kingdoms. The king of Babylonia at this time was Adad-shum-nasir.
2. King Ashur-nirari III is succeeded by Enlil-kudur-usur (1207-1203).
The growing hostilities boil over and into war as the newly crowned king of Assyria, Enlil-kudur-usur, wages war with the Kassites, challenging Babylonia's power. A key battle towards this conflict's finale sees king Adad-shum-nasir meet Enlil-kudur-usur on the field where both kingly commanders are slain.
3. King Enlil-kudur-usur is succeeded by Ninurta-apal-ekur (1202-1176).
Upon the deaths of their respective kings, both nations desperately scrambled for new leadership, and it is here that the Assyrians were granted a longlasting ruler. King Ninurta-apal-ekur may have inherited the conflict against Babylonia but at first struggled quite a bit. Early on, he managed to retreat back to Assyria in order to reinforce his army but was pursued by the new Babylonian king, Meli-Shipak II (1202-1188), who attempted to conquer the Assyria at its heartland.
Ninurta-apal-ekur was able to regroup his forces just in time to successfully defend Assyria's territory and expel Meli Shapak II and his Kassites back home. While both sides engaged in an uneasy peace for a few years, the strong enmity between both civilizations was still too great.
For many more years, Assyria and Babylonia engaged in a series of violent conflicts, passed on from one ruler to the next, unto the subsequent generations of the next day. Assyria's expansion finally met its match in the Kassites who refused to bend their knees to the Assyrians ever again.