Complex sentence: In the winter of 1941, before "Wannsee" but after "Barbarossa" the Nazi demands for forced labor greatly intensified, therefore Himmler and Heydrich approved the Jewish substitution policy in Upper Silesia and in Galicia under the "destruction through labor" doctrine.
Split into simpler sentences: In the winter of 1941, before "Wannsee" but after "Barbarossa", the Nazi demands for forced labor greatly intensified. Therefore, Himmler and Heydrich approved the Jewish substitution policy in Upper Silesia and in Galicia under the "destruction through labor" doctrine.