Adad-nirari I (aka Adad-narari I) (1307-1275 BC) grew Assyrian power and described himself as defeater of the feocious ones, turning Hanigalbat (Assyria's term for the Mitanni kingdom) into a vassal before annexing it completely (including Washukanni, the old Mittanni capital). However, Adad-narari I was unsuccessful against the Hittites and lost large parts of Mesopotamia to them; after trying to form a brotherly alliance with the Hittites, their king snidely responded; Why should I write to you about brotherhood? Were you and I born of the same mother?.
Assyria now controlled the whole western and northern territory within the defensible boundaries of the Euphrates and Tigris, giving Assyria hegemony over the riverine trade routes. To the south, just east of the Tigris, the boundary between Assyria and Babylonia was formed by either the Lower Zab, Adhaim or Diyala rivers. Adad-narari's hard stance on this boundary led to celebratory epic, one of Assyria's first native literary works. However, this area was battled over innumerable times and fluctuated according to levels of Assyro-Babylnonian power.