Aided by the Byzantine Empire 's preoccupation with the long and bloody wars with Sassanid Persia in the east , and with the Avar Khaganate in the north , the Slavs raided and settled almost at will .
From 578 , Slavic raids reached Thessaly and southern Greece .
Aided by the Byzantine Empire 's preoccupation with the long and bloody wars with Sassanid Persia in the east , and with the Avar Khaganate in the north , the Slavic raided and settled almost at will .
From 578 , Slavic raids reached Thessaly and southern Greece .