For over 400 years, Sassanid Persia was the greatest state in Asia. To the east, the Kushan Empire was already in decline. The only strong opponent of Iran was the Roman Empire in the west. Military competition for influence in northern Mesopotamia, Armenia and the Caucasus region dominated Iranian–Roman relations, orienting the strategic activities of the early Sassanids to the western fringes of the empire. The breakthrough came in the mid-fourth century, with the emergence of the Kidara Huns in the east. Iran faced a ‘strategic dilemma’: it was crucial to avoid wars on multiple fronts. The Hephthalites or White Huns, became the most important enemy of the Sassanians until the end of the following century; the adoption of such a strategic paradigm enforced the maintenance of peace with the Roman Empire in the west. However, the Sassanian ruler, having secured the eastern territories, was able to move against Iran’s age-old enemy, Rome, this way beginning a period of wars in the west that, with few interruptions, lasted almost until the collapse of the Persian state. Defending such an enormous area was a challenge, as was preventing it from centrifugal tendencies, typical for multi-ethnic states. Despite these factors, the Iranian state managed to assure the territorial integrity of its core areas for four centuries. The tool to achieve this was the army – mobile, efficient, disciplined and motivated.