Book excerpt: The East Romans of Byzantium and the Sasanian Persians competed as geopolitical rivals for over four centuries between 224 to 628 AD. Through a series of intractable conflicts these two great empires would develop a dual hierarchy that sought to divide the known world between them. Despite competing claims to universal rule, mutual spheres of interest arose as both empires sought to create rules, norms, and standard practices of diplomatic behaviour to regulate their inter-imperial rivalry. Defined by contemporaries as the 'Two Eyes' of the Earth, this suzerain order aimed to hierarchically organize those considered as 'barbarians'. This period of late antiquity is rarely considered within the discipline of International Relations (IR) but, through an English School approach, Blachford examines the diverse suzerain order of late antiquity as 'barbarous' nomadic tribes challenged the hierarchical ambitions of two rival empires who both claimed a unique role in the maintenance of world order.