This article covers the legal status, condition, and functions of the Armenian troops, which fought within the Byzantine army on behalf of the Persian king Khosrow II Parwiz in 591 during his clash with a Persian usurper Bakhram Chobin. The author turns his attention to the sources, which had been neglected in previous historiographic studies. The sources ate the accounts of Arabic-speaking historians al-Dinawari and al-Tabari, who lived in the 9th - early 10th century. Having compared their evidence and the narratives of the Greek (Theophylactus Symocatta) and Armenian (Sebeos) historical chronicles, the author has discovered a social origin of the Armenian troops, which were a part of the Byzantine army. In addition, the author has traced the way of their territorial movements and studied in succession how the functions and status of these Armenian units changed over the time. In regards to the methods, the author highlights the usage of the historical-chronological narrative, i. e. the analysis of event history, when events are arranged and presented in strict chronological order. This method has allowed seeing the events in consecutive manner: how the status and functions of Armenian ethnic units within the Byzantine army changed before and after 590-591. The main conclusion of this contribution is that the Armenian units, which fought for the sake of Khosrow II Parwiz within the Byzantine troops, represented the retinues of nakharars, who were the chiefs of Armenian noble families, but first these nakharars located on a territory of Persia, not Byzantium. The supreme and general leader of such retinues was a powerful nakharar Moushel Mamikonean, who refused to accept Bakhram and as a result joined the Byzantine troops, which came to Persia at the request of Khosrow in order to destroy Bakhram. After Bakhram was defeated, Moushel and the nakharars subordinated to him, moved and resettled in Byzantium. However, because of the mistaken policy the Byzantine Emperor Mauricius led towards the Armenian noble, Moushel and his nakharars left Byzantium and fled back to Persia. The departure of the Armenian nakharars generated a serious weakening of Byzantine troops, which stood in Armenian provinces of Byzantium, and this process, in turn, caused the hard defeats of Byzantines from the Persians during 603-607, when Mauricius was already murdered as a result of the internal Byzantine military unrest.