The Katyn massacre was a mass execution of Polish citizens by the order of Soviet authorities in 1940. About 8,000 of those killed were reserve officers taken prisoner during the 1939 Invasion of Poland, but the dead also included many civilians who had been arrested for being "intelligence agents and gendarmes, spies and saboteurs, former landowners, factory owners and officials". Since Poland's conscription system required every unexempted university graduate to become a reserve officer, the Soviets were thus able to round up much of the Polish, Jewish, Ukrainian, Georgian and Belarusian intelligentsias of Polish citizenship. The 1943 discovery of mass graves at Katyn Forest by Germany, after its armed forces had occupied the site in 1941, precipitated a rupture of diplomatic relations between the Soviet Union and the Polish government-in-exile in London. The Soviet Union continued to deny responsibility for the massacres until 1990, when it acknowledged that the NKVD had in fact committed the massacres and the subsequent cover-up. The Russian government has admitted Soviet responsibility for the massacres, although it does not classify them a war crime or an act of genocide, as this would have necessitated the prosecution of surviving perpetrators, which is what the Polish government has requested.