The return of contemporary great power rivalry has renewed interest in the idea of a ‘Thucydides trap’ within IR and security studies. Missing, however, are discussions of great power conflict between Thucydides and 1648. The following paper seeks to open new ground by examining the bipolar rivalry between Byzantium and the Sasanid empire. This four century rivalry rested upon a fragile buffer zone along the eastern Roman frontier. Its longevity was based on a stable offense-defense balance that eventually collapsed in the seventh century. The Thucydides trap narrative presents a bipolar clash with an air of inevitability, this interdisciplinary case study however, develops a more nuanced narrative, which shows how the Byzantium and the Sasanid rivalry existed across a buffer zone that only collapsed once imperial ideologies led to competing irreconcilable visions of international order.