The Byzantine and Sassanid Empires divided Armenia in 387 and 428 AD. Western Armenia was part of Byzantine rule, whilst Eastern Armenia (Persianized Armenia) was part of the Sassanids. Even after the Bagratid Armenian Kingdom was created, parts of historic Armenia and Armenian-inhabited areas were still under Byzantine rule.
References
↑Rivoira, Giovanni Teresio (1918). Moslem Architecture: Its Origins and Development. Oxford University Press. p. 188.