Kassite Babylonia counts among the great powers of the Late Bronze Age Near East. Its kings exchanged diplomatic letters with the pharaohs of Egypt and held their own against their Assyrian and Elamite neighbors. Babylonia’s internal workings, however, remain understood in their outlines only, as do its elite’s expansionary ambitions, the degrees to which they may have been realized, and the nature of ensuing imperial encounters. This is especially the case for the region to the northeast, where the Mesopotamian lowlands meet the Zagros piedmonts in the Diyala River valley and where a series of corridors of movement intersect to form a strategic highland-lowland borderland. In this paper, we present critical new results of regional survey in the Upper Diyala plains of northeast Iraq and excavations at the Late Bronze Age site of Khani Masi. Not only do our data and analyses expand considerably the known extent of Babylonia’s cultural sphere, but also the monumental character of Khani Masi and its wider settlement context prompt a fundamental rethinking of the nature and chronology of Babylonian presence in this transitional landscape. As such, this paper contributes an important new case study to the field of archaeological empire and borderland studies.