The Katyń Massacre is the name given to the mass murder of 21,768 Polish army officers and members of the intelligentsia and middle classes by Soviet NKVD killers at the Katyn Forest in western Russia and other locations in Poland, Belarus, Russia and Ukraine. The killings were directly ordered by Stalin, on the suggestion of his secret police chief Lavrenty Beria, following the Soviet annexation of eastern Poland in 1939. The crime was discovered and announced to the world by the invading German forces in 1943, and led to the breaking-off of diplomatic relations between the London-based Polish Government-in-exile and the Soviet Government. The Soviet Union denied responsibility for the murders until 1990.
In August 1939, Nazi Germany and the Soviet Union signed the Molotov–Ribbentrop Pact, a non-aggression treaty between the two countries. As part of the agreement, the two nations determined respective spheres of influence in Eastern Europe, including a division of Poland (despite the existence of a Soviet–Polish Non-Aggression Pact signed in 1932).
Nazi Germany invaded Poland on September 1, 1939. Despite the peace between them, the alliance between Russia and Germany was a suspicious one, largely due to massive political and ideological differences between the Socialist Nazi government and the Communist Russians. Soviet leader Josef Stalin did not trust the Germans to halt their invasion within the predetermined sphere of influence stated in the Molotov–Ribbentrop Pact. Because of this, and seeking to protect its own interests there, Russia invaded Poland on September 17.
Already defeated by the Nazi ''Blitzkrieg'' invading from the east, Polish soldiers fleeing the Germans were easily captured Soviet forces advancing from the west. The Red Army took over 40,000 Polish soldiers hostage, including approximately 8,500 officers. These prisoners (mostly men) were herded into prison camps in the towns of Yuzhe, Kozelsk, Kruzkha, Ostashkov, Kitaygorod, Starobielsk and Kryazovets.
Within several days of the invasion, Soviet officials were developing plans handling of Polish prisoners, including disposal of people likely to be disruptive to Soviet rule, including teachers, doctors, lawyers and other educated or "counterrevolutionary" elements. Military officers were also targeted, since a leaderless Polish military would pose no threat to the Russians.
In March 1940, Stalin ordered Lavrentiy Beria, head of the NKVD (Soviet Secret Police) to execute approximately 20,000 Polish captives. 
The executions carried out in Katyn were ruthless and efficient. Most prisoners had their hands bound and were either led to a mass grave, where the were subsequently shot in the back of the head or neck. In some cases, especially high-ranking officers were led into a nearby slaughterhouse where their identity could be firmly established with identity cards and files. They were then led into an adjoining room where they were also shot in the back of the head. The bodies were dumped in large trucks and driven to mass grave sites.
Among those murdered at Katyn were an admiral, two generals, 24 colonels, 79 lieutenant colonels, 258 majors, 654 captains, 17 naval captains, 3,420 Non-Commissioned officers, seven chaplains, three landowners, a prince, 43 officials, 85 privates, and 131 refugees.
Among the civilian victims were 20 university professors, 300 physicians, several hundred lawyers, engineers, and teachers; and more than 100 writers and journalists as well as about 200 pilots.
Discovery and aftermath
In June 1941, Nazi Germany commenced Operation Barbarossa, a large-scale military invasion of Russia. At that point, it was the largest military offensive in recorded history, involving over 3 million troops. 
Nazi forces gained ground quickly, advancing over 300 miles (321 kilometers) within a week. Part of that territory included the city of Smolensk and the surrounding area, where the bulk of the executions took place. However, the murders remained undiscovered another two years.
In April 1943, German troops unearthed a mass grave of over 4,000 Polish officers. Nazi Propaganda Minister Joseph Goebbels quickly seized the opportunity to use this news to discredit Stalin and unbalance the already-delicate alliance between the Western Powers and the Soviet Union. German radio revealed news of the atrocity, and the Germans garnered further publicity by convening a panel of forensic experts to examine the bodies. Numerous photos of the mass graves were published as well.
Russian authorities quickly denied any responsibility, instead blaming the Germans for the atrocity. The Soviets clung to this position even after forensic examinations quickly established the bullets to be of Russian manufacture and that the executions took place in 1940, when the area was still under Russian control. The Soviet Union denied responsibility for the murders until 1990.
In popular culture
The 2004 Japanese game Metal Gear Solid 3: Snake Eater referenced the events of the Katyn Forest Massacre and was made relevant due to the main villain Colonel Volgin being involved in the massacre.
The 2007 Polish film Katyn, which explored the events leading up to the executions, and the lives of several victims' loved ones in Postwar Poland, was nominated for the Academy Award for Best Foreign Film.