Perhaps the Byzantine Empire's most significant achievement during its thousand-year history was the civilizing of its northern neighbors, the Slavs, through Christianization and political tutelage. The process was not entirely altruistic; rather, it was designed to secure the empire's northern border against the Slavs and other peoples. This border shifted over time, sometimes embracing the entire Balkan peninsula as far as the Danube to modern Croatia, sometimes comprising little more than the Thracian littoral and a few isolated areas in the south of the Greek peninsula.