1943 April 13th. Katyn Forest Massacre.

• 1943 – World War II: The discovery of mass graves of Polish prisoners of war killed by Soviet forces in the Katyń Forest Massacre is announced, causing a diplomatic rift between the Polish government in exile in London from the Soviet Union, which denies responsibility.The massacre was prompted by NKVD chief Lavrentiy Beria's proposal to execute all captive members of the Polish Officer Corps, dated 5 March 1940, approved by the Soviet Politburo, including its leader, Joseph Stalin. The number of victims is estimated at about 22,000. The victims were murdered in the Katyn Forest in Russia, the Kalinin and Kharkiv prisons and elsewhere. Of the total killed, about 8,000 were officers taken prisonerduring the 1939 Soviet invasion of Poland, another 6,000 were police officers, and the rest were arrested Polish intelligentsia the Soviets deemed to be "intelligence agents,gendarmes, landowners, saboteurs, factory owners, lawyers, officials and priests".
• The government of Nazi Germany announced the discovery of mass graves in the Katyn Forest in 1943. When the London-based Polish government-in-exile asked for an investigation by the International Red Cross, Stalin immediately severed diplomatic relations with it. The Soviet Union claimed the victims had been murdered by the Nazis, and continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the perpetration of the killings by the NKVD, as well as the subsequent cover-up by the Soviet government.
• An investigation conducted by the Prosecutor General's Office of the Soviet Union (1990–1991) and the Russian Federation (1991–2004), confirmed Soviet responsibility for the massacres, but refused to classify this action as a war crime or an act ofgenocide. The investigation was closed on the grounds that the perpetrators of the massacre were already dead, and since the Russian government would not classify the dead as victims of Stalinist repression, formal posthumous rehabilitation was deemed inapplicable.
  In November 2010, the Russian State Duma approved a declaration blaming Stalin and other Soviet officials for having personally ordered the massacre.