The fact that the themes were not the result of one legislative act meant that each theme had its own history, sometimes a rather long one. The problem of the origin of themes can be solved only by special research on each individual theme. Kulakovskys writings are of interest in this connection. The military measures taken by Heraclius after his victory over Persia were, he believed, the point of departure of the new administrative regime. Brehier supported Kulakovsky in this view. Armenia may be an example of the militarization of the empire under pressure of the Persian danger, for when Heraclius reorganized Armenia, he appointed no civil administrator. The authority was purely military. The theme system, then, was merely the application to other provinces of the regime instituted in Armenia. Th. Uspensky called attention to the Slavs. When they inundated the Balkan peninsula about the time of the theme formation, he said, they contributed to the formation of the theme organization in Asia Minor by supplying a considerable number of volunteers for the colonization of Bithynia. This statement is to be taken with caution, however, for there is no evidence of a mass Slav immigration into Asia Minor before the transporting of 80,000 Slavs to Opsikion under Justinian II at the end of the seventh century.
It is definitely known that for defense against the oncoming danger there were established in the East in the seventh century the following four large military districts, later called themes: 1) Armeniaci (Armeniakoi) in northeast Asia Minor bordering on Armenia; 2) Anatolici (Anatolikoi, from the Greek word Anatoli, Ἀνατολή, the east); 3) the imperial God-guarded Opsikion (Greek Ὀψίκιον, Latin, obsequium), in Asia Minor near the Sea of Marmara; and 4) the maritime thema Caravisionorum, called later, perhaps in the eighth century, Cibyrrhaeot (Cibyraiot), on the southern shore of Asia Minor and in the neighboring islands. The first two, occupying the entire middle portion of Asia Minor from the borders of Cilicia in the east to the shores of the Aegean Sea in the west were intended to serve as a protection against the Arabs. The third was to shield the capital from external enemies. The fourth, the maritime theme, was intended as a defense against the Arabian fleet.
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Reference address : https://www.ellopos.net/elpenor/vasilief/theme-organization.asp?pg=3