In accordance with the Molotov-Ribbentrop Pact, National Socialist Germany and the Union of Soviet Socialist Republics invaded and divided Poland in September 1939 as de facto allies. After a chaotic winter stabilizing eastern Poland (Western Belarus and western Ukraine today), the Soviet Union began an organized and deliberate campaign to ethnically cleanse Poles from its conquered territory. In early 1940, the NKVD (the Soviet secret police, the forerunner of the KGB) was holding over 500,000 Polish prisoners. On 5 March 1940, Laventry Beria, the head of the NKVD proposed the execution of all possible Polish leadership in captivity. The execution order extended to any person formerly of the Polish Army officer corps and any civilian leadership, including anyone who showed any signs of leadership ability. The proposal was approved by the Soviet Politburo and Josef Stalin.

Starting 3 April 1940, hundreds of Polish Army officers, government workers, land owners, school teachers, university professors, police officers, “intelligence agents”, lawyers, scientists, Polish Jews, factory managers, writers and publishers, business owners, Boy scouts and scoutmasters, and priests and clergy were murdered every night in their camps. The subject was usually grabbed from a prison gathering, had his or her credentials checked against a list of undesirables, led to a cell lined with sandbags, forced to kneel, and then shot in the back of the head or neck. The shots were muffled by the sandbags and the use of machinery and fans to prevent rioting among the prisoners. The executions were usually carried out using .25 ACP Walter Model 2 pistols obtained from Germany through prewar trade deals. Since the Walter Model 2 had significantly less recoil than the Russian made 7.62 Nagant M1895 revolvers in Soviet service at the time, more executions could be made in less time. Soviet executioners found that the Nagant’s recoil began to make executions difficult after the first dozen; there was no such problem with the smaller, but equally effective round of the Walter. The executions were carried out in camps all over Western Russia, Eastern Poland, Belarus, and Ukraine. The corpses were carried onto trucks and then taken to mass graves deep in the forest.

In June 1941, National Socialist Germany stabbed its erstwhile ally in the back and invaded the Soviet Union. In late 1942, captive Polish railroad workers heard from locals that mass graves of Polish soldiers were located in the Katyn Forest. A few months later, a German intelligence officer became aware of the rumor and had it investigated. A mass grave filled with 3000 bodies was discovered on Goat Hill in the Katyn Forest. Further investigation found more mass graves in the area, totaling more than 22,000 bodies. Nazi propaganda minister Josef Goebbels seized upon the discovery to show “the horrors of Bolshevism and Anglo-American subservience to it”. The European Red Cross formed the “Katyn Commission” of forensic experts, neutral journalists, and Allied prisoners of war, who were brought in to observe to investigations and excavations. The Soviets denied the accusation that they were responsible. After the Soviets overran the sites in 1943, the London based Polish Government-in-Exile asked Stalin to investigate. Stalin immediately broke off relations with the Poles and accused them of being Nazi collaborators.

With the Soviet Union bearing the brunt of the fighting against Nazi Germany, the United States and Britain accepted the official Soviet explanation that the mass graves were Polish construction workers murdered by the Nazis in 1941. President Roosevelt had his own commission look into the massacre and when it came back conclusively that the Soviets were responsible, Roosevelt ordered the report destroyed and the lead investigator exiled to American Samoa for the rest of the war. The Soviets denied responsibly until 1990. After the Cold War, about a dozen sites similar to the one in Katy Forest were identified, a testament to the extent of the murders of Polish leadership by the Soviet Union in 1940.

On 10 April 2010, Polish Prime Minister Lech Kaczyński, his wife, and Poland’s top military and political figures flew to Smolensk, Russia to attend the 70th anniversary memorial ceremony of the Katyn Massacre. On approach to the Smolensk airport, their plane crashed and everyone on board was killed. Russian Prime Minister Vladimir Putin was put in charge of the investigation and he concluded “pilot error”. Poland disputes the findings. As of April 2020, Russia has yet to turn over any evidence, including the plane’s wreckage and black boxes, for independent Polish or international investigation.