the Germanic tribes almost completely wipe out christianity (including arianism) west of sardinia, the Persians have converted from Zoroastrianism to nestorian Christianity, but their empire has a large number of Eastern Orthodox christians, especially the Assyrians. Then, theodora dies before the visigothic queen does, and Justinian re-marries her, unifying the Byzantine empire with Italy. All this makes the Sassanid empire more subjugatable and the west, less, this all motivates him to go east, instead of west.