The `` Intelligenzaktion '' was a major step to implementing Action `` Sonderaktion Tannenberg '' ( Operation Tannenberg ) , the installation of Nazi policemen and functionaries — from the SiPo ( composed of the Kripo and Gestapo ) , and the SD — to manage Hold the occupation and facilitate Cause_to_make_progress the realisation of `` Generalplan Ost '' , the German colonization of Poland .