Perhaps the Byzantine Empire's most significant achievement during its thousand-yearhistory was the civilizing of its northern neighbors, the Slavs, through Christianization andpolitical tutelage. The process was not entirely altruistic; rather, it was designed to secure theempire'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 ofthe Greek peninsula.