The Slavs and the Old Prussians. Poland in the Early Medieval Period / Славяне и Древние Пруссы. Польша в период Раннего Средневековья.: The logic of Kostrzewski’s argumentation was so convincing that when Nazis occupied Poland in 1939 they placed him on the top of the list of Polish intellectuals planned for exter¬mination. Fortunately, Professor Kostrzewski managed to go into hiding a