Germany conquered the Slavic nations during World War II. As the Nazi ideology considered Slavs Untermenschen, most Slavs were subjected to enslavement and genocide. German settlers moved into former Slavic lands with the exception of the Czechs and Croats, which were regarded as western or Germanic despite their Slavic roots, and merely occupied militarily. Bulgaria, which was a German ally, was also exempted from the horrors visited on most Slav nations.
The Slavs were the latest in a growing number of Barbarian peoples invading the Byzantine Empire. They were subjects of Avars, a nomadic people which had established a powerful khaganate north of the Danube River, and were trying to expand southward at the Byzantines' expense. The city of Thessalonica was besieged by a great army of Slavs whom their Avar overlords relentlessly drove to attack again and again, even when they suffered great losses. Along with the human Slavs came their aggressive gods, demigods and various supernatural creatures, posing a mortal threat to both human Byzantines and to the old-established Greek supernatural creatures such as satyrs and Centaurs, already greatly diminished by the advent of Christianity; these were driven into a fragile alliance with the people of Thessalonica, which proved the Slavs' undoing.