The course of the Islamic conquests of the Middle East, particularly the arena of the initial conquests in Bilad ash-Sham and al-Iraq, has been the subject of many studies. Many of these previous studies have focused on the 10th century narratives of the conquests, sometimes augmented by modern studies of Arabic epigraphy and correctives offered by the Syriac, Armenian, and Persian sources. However, the majority of the attention has been on the background of the conquests in Mecca and Medina and within the nascent Muslim community (Donner, Crone, etc.). Even when a greater context has been considered and incorporated into the studies (Hoyland), this has been mostly in the context of the former Byzantine territories in Syria and North Africa. The sudden and complete fall of the Sasanian Empire, the subsequent 'lands of the Eastern Caliphate' has been of less interest to many studies. This is mainly because the number of languages required for contextualising the Sasanian fall and the Islamic conquests of the Iranian lands has been daunting, and that information such as archaeological evidence has been scarce. Newer information and updated evaluation of the sources, however, make it possible to understand the conquests from a Sasanian viewpoint as well. This paper, relying on textual, archaeological, and numismatic evidence, studies the late Sasanian army and its engagement in the conquest of the Near East in the wars of 602-628. It then places these forces within the context of the 634-642 conquests that follow the Sasanian-Byzantine wars, aiming to particularly understand the role of mercenary forces. By providing a more nuanced narrative of the Sasanian-Byzantine Wars and focusing on the socio-economic aspects of the military engagement, the paper would suggest a more local theatre of engagement for the early Muslim conquerors and their paths of advancement in the former Sasanian and Byzantine lands.