The Slavs and the Old Prussians. Poland in the Early Medieval Period / Славяне и Древние Пруссы. Польша в период Раннего Средневековья.: After 1945 J. Kostrzewski’s “Proto-Slav” theory took on an entirely new dimension, not foreseen by him. The Polish communist authorities adopted it as a highly useful propaganda tool. They wished to prove that much of Slask, parts of western Wiclk