In Byzantine history the organization of the themes is usually connected with the epoch of the Heraclian dynasty. The organization of the themes means that peculiar provincial organization, prompted by the conditions of the times, whose distinguishing feature was the growth of the military power of the provincial governors, and finally their complete superiority over the civil authorities. This process was not sudden but gradual. For a long time the Greek word theme (θέμα) meant a military corps stationed in a province, and only later, probably in the eighth century, was it applied not only to the military detachment, but also to the province where it was stationed. Thus it began to be applied to the administrative divisions of the Empire.
The main Byzantine source on the problem of the themes is the work On Themes, written by Constantine Porphyrogenitus, the emperor of the tenth century, and hence dating from a period much later than the epoch of the Heraclian dynasty. This work has also the disadvantage of being based in some places on geographical works of the fifth and sixth centuries, used very superficially or copied verbatim. But although this work does not give much information on theme organization in the seventh century, it does connect the beginning of the system with the name of Heraclius. The Emperor said: Since the reign of Heraclius the Libyan (i.e. African), the Roman Empire has become reduced in size and mutilated both from the east and from the west. Very interesting, though not yet fully explained, material on this problem is found in the works of the Arabian geographer's Ibn-Khurdadhbah (Khordadhbeh), of the first half of the ninth century, and Kudama, of the early tenth century, though these men, of course, were not contemporaries of the Heraclian epoch. For the study of the earlier period of the theme system, historians have made use of occasional remarks of chroniclers and especially of the Latin message of Justinian II to the pope, dating from the year 687, regarding the confirmation of the Sixth Ecumenical Council. This epistle contains a list of the military districts of that period, not yet referred to as themes, but denoted by the Latin word exercitus (army). In historical sources of that time the Latin word exercitus and the Greek word στρατὸς or στράτευμα sometimes were often used in the sense of a territory or province with military administration.
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