The Katyn massacre was a series of mass executions of nearly 22,000 Polish military officers and intelligentsia prisoners of war carried out by the Soviet Union, specifically the NKVD ("People's Commissariat for Internal Affairs", the Soviet secret police) in April and May 1940. Though the killings also occurred in the Kalinin and Kharkiv prisons and elsewhere, the massacre is named after the Katyn Forest, where some of the mass graves were first discovered by German forces.

The massacre was initiated in NKVD chief Lavrentiy Beria's proposal to Joseph Stalin to execute all captive members of the Polish officer corps, which was approved by the Soviet Politburo led by Stalin. Of the total killed, about 8,000 were officers imprisoned during the 1939 Soviet invasion of Poland, another 6,000 were police officers, and the remaining 8,000 were Polish intelligentsia the Soviets deemed to be "intelligence agents and gendarmes, spies and saboteurs, former landowners, factory owners and officials". The Polish Army officer class was representative of the multi-ethnic Polish state; the murdered included ethnic Poles, Ukrainians, Belarusians, and Jews including the chief Rabbi of the Polish Army, Baruch Steinberg.The government of Nazi Germany announced the discovery of mass graves in the Katyn Forest in April 1943. Stalin severed diplomatic relations with the London-based Polish government-in-exile when it asked for an investigation by the International Committee of the Red Cross. After the VistulaOder offensive where the mass graves fell into Soviet control, the Soviet Union claimed the Nazis had killed the victims, and it continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the killings by the NKVD, as well as the subsequent cover-up by the Soviet government.

An investigation conducted by the office of the prosecutors general of the Soviet Union (19901991) and the Russian Federation (19912004) confirmed Soviet responsibility for the massacres, but refused to classify this action as a war crime or as an act of mass murder. The investigation was closed on the grounds that the perpetrators were dead, and since the Russian government would not classify the dead as victims of the Great Purge, formal posthumous rehabilitation was deemed inapplicable. In November 2010, the Russian State Duma approved a declaration blaming Stalin and other Soviet officials for ordering the massacre.

The falsified Soviet version of the events has become known as the "Katyn lie", a term coined in reference to the "Auschwitz lie".

The Political Bureau of the Central Committee of the Communist Party of the Soviet Union (or Politburo, Russian: Политбюро, IPA: [pəlʲɪtbʲʊˈro]; abbreviated: Политбюро ЦК КПСС, Politbyuro TsK KPSS) was the highest policy-making authority within the Communist Party of the Soviet Union. It was founded in October 1917, and refounded in March 1919, at the 8th Congress of the Bolshevik Party. It was known as the Presidium from 1952 to 1966. The existence of the Politburo ended in 1991 upon the breakup of the Soviet Union.