The death of General Sikorski on July 4, 1943 coincided with the disclosure by the Germans of the massacre of Polish officers in Katyn Forest by the Soviet NKVD. The appeal made to the International Red Cross by the Polish government-in-exile for an investigation of the atrocity was used by Stalin as a pretext to break off Polish-Russian diplomatic relations. This was a critical moment; Poland's historical and cultural heritage were now at risk from both the Soviet Union and Nazi Germany. After displacing the Red Army from the eastern regions of Poland, the Nazis occupied the entire country; their aim was the destruction of Polish scholarship and cultural heritage. This plan was being implemented through the extermination of the middle class, the intelligentsia, systematic plundering of libraries and museums, and by forbidding cultural, educational, and artistic activities. These actions were reinforced by mass arrests and deportations to concentration camps (such as Sonderaktion Krakau), which led to the murder of nearly all faculty of the Jagiellonian University. The closure of the Polish Academy of Sciences and other research and academic institutions left no doubt as to the Germans' intentions.