In April 1943, the occupying German troops discovered in the Katyn Forest near Smolensk in the Soviet Union, eight mass graves and announced that a total of 4,443 corps that were recovered from those graves, had apparently been shot from behind and then piled in stacks and buried. It was also alleged by the Germans that the corps are related to the remains of 22,000 Polish citizens, including the Army officers, civil servants, police officers, priests and intellectual leaders, captured after the Soviet Union’s aggression on Poland and interned at the prisoner-of-war camps at Kozielsk, Starobielsk and Ostashkow and subsequently massacred by the Soviet NKVD, the Soviet secret police, also known as the People's Commissariat for Internal Affairs.

In retaliation to the charges, the Soviet government vehemently refuted the allegation and claimed that those Poles had been engaged in construction work in the west of Smolensk in 1941 and were killed by the invading German army, after overrunning that area in August 1941. However, investigations of the Katyn corpses, both by the Germans and the Red Cross, produced firm physical evidence that the massacre took place in early 1940, at a time when the area was still under Soviet control.

It all started on 23 August 1839, when Nazi Germany and the Soviet Union concluded their Nonaggression Pact, which included a secret settlement for the partition of Poland. Closely followed by the pact, Germany invaded Poland from the west on the 1st day of September 1939, accusing Poland of persecuting ethnic Germans living in Poland and also falsely alleging that Poland was planning, with its allies, Great Britain and France, to encircle and dismember Germany. Simultaneously, just two weeks following the German attack, the Red Army attacked and occupied the eastern half of Poland. Considering the lack of support of the allies in the hour of need as the stabbing in the back, the Polish government decided to evacuate the highest authorities of the Republic of Poland abroad and at the same time, an unclear directive was issued by the Commander-in-Chief, ordering the troops to get to Hungary or Romania and avoid fighting the Red Army. However, despite some Polish units and garrisons decided to overrule the order and took part in uneven battles against the aggressor, the imminent disaster was inevitable, due to the huge disproportion. As a result, the independent Polish state was eliminated and its territory was seized by the Third Reich and the Soviet Union.

At the same time, the Soviets captivated around 240 to 250 thousand Poles, including about 10 thousand officers of the Polish Army and without adhering to all the international norms and conventions, they handed over the prisoners to the NKVD security services.

Three special camps were set up for the prisoners. While the senior state and military officials were hoarded in the Starobielsk and Kozielsk camps, soldiers of the Border Protection Corps, the Prison Guards, the Border Guards, along with the officers of the State Police, intelligence and counterintelligence, were dumped in the in Ostashkow camp. They had to stay in those overcrowded camps, day after day, in extremely distressing, unhealthy, unclean and subhuman conditions, without adequate water and food.

In addition, each of the prisoners was inquired and interrogated, day after day, about their political views, social and professional position, assets, even their associations with foreign countries and skills in any foreign language. Finally, their files were forwarded to the Special Council, tasked to decide on the fate of the prisoners of war and other captivated people held by the NKVD.

On the other side of the story, as the Germans started enacting their infamous tortures and elimination of the Polish population, the Russians were also embarking on a similar campaign of brutal suppression. According to the proposal submitted by Lavrentiy Beria, the People’s Commissioner of Internal Affairs of the Soviet Union, Józef Stalin personally decided to murder Polish prisoners of war, which was endorsed and duly signed by the members of the Politburo of the Central Committee of the Communist Party of the Soviet Union. However, they wanted the whole action to be carried out quickly, quietly and efficiently, only under the knowledge of a few persons. It is believed by many that the Russian eagerness to slaughter Poland's military leadership was not simply born out of savagery, but because of Stalin’s belief that such ruthless tactics would permanently weaken the morale of the Poles and make them a much easier people to subdue. Another motive behind the mass murder was probably the idea of depriving the Polish nation of its leaders and intellectual elite. Nevertheless, the first death letters reached the camps in Starobielsk, Kozelsk and Ostashkow in early April and to the prisons in Minsk, Kyiv, Kharkiv, and Kherson and the torture houses in Kharkiv and Kalinin around 20 April 1940.

While the executioners were being trained in killing people with a single shot in the back of the head, the prisoners were led out of the camp gate in groups of two, put all of them into the waiting prison wagons, eight men in each compartment and locked barred the corridor doors with special locks. Prisoners were shifted from the camps to the places of execution. From Kozielsk camp they were transported by cattle wagons to Smolensk, from Starobielsk to Kharkiv and from Ostashkow to Kalinin. Eventually, the convoy arrived at a large church in Kozielsk in north-eastern Poland in early November, where some 4,000 other officers were quartered in a Prisoner of War camp. Executions were carried out with surgical precision, only at night, in strict secrecy. They were brought to the cellars, where, in a solitary and muted room, they were murdered with a single shot in the back of the head. Those who struggled were tied up or blindfolded or had coats thrown over their heads. Initially, the Russian executioners used Nagant revolvers, which were later replaced by the German Walthers 7.65, due to their lighter weight and better performance. The shooting spree ended at dawn, when the corpses were transported by trucks to solitary places with already dug pits and the gravediggers were replaced by excavators to level the ground.

Tied up hands of one of the murdered Polish officers

The world was unaware of the genocide until June 1941, when the Germans invaded the Soviet Union and the Polish government-in-exile, located in London, agreed to cooperate with Soviet Russia against Nazi Germany. However, as it became necessary to form a Polish army on Soviet territory, the Polish general Wladyslaw Anders, who was organizing the army, requested that 15,000 Polish prisoners of war whom the Soviets had once held at Smolensk camp be transferred to his command. But he was informed by the Soviet government in December 1941 that almost all those prisoners had somehow escaped to Manchuria and could not be located. The fate of those missing prisoners remained a mystery till 13 April 1943, when the Germans announced the discovery of the mass graves of Polish officers in the Katyn forest near Smolensk, allegedly murdered by the Soviet NKVD.  Although the Soviet government refuted the charges and claimed that the Poles were killed by the invading Germans in August 1941, the Polish government-in-exile in London requested the International Committee of the Red Cross to examine the graves and also asked the Soviet government to provide sufficient official reports on the fates of the remaining missing prisoners. Refusing to oblige the Polish government in London, the Soviet government broke diplomatic relations with them on 25 April 1943 and set about establishing a Polish government-in-exile, composed of only the Polish communists.

After the war, the case of Katyn was raised before the Nuremberg Tribunal, but it did not go to trial as the guilt of the Germans could not be proven. Over the following decades, the Soviets tried to erase the memory of the Katyn massacre, but for the Poles, Katyn became a symbol of the victims of Stalinism. For the people of Poland, the Katyn Massacre was an attack on the Polish nation, aimed to prevent the rebuilding of a sovereign Polish state. However, the claim of the Soviet leaders that the Katyn massacre was committed by the invading Germans in 1941 was accepted without protest by the successive Polish communist governments until the late 1980s, when a noncommunist coalition government came to power in the country in March 1989 and officially shifted the blame for the Katyn Massacre from the Germans to the Soviet secret police.

The Katyn massacre is classified as a crime against humanity, a crime against peace, a communist crime and a crime of genocide. However, the revealing of the truth about Katyn inside the USSR was only made possible by perestroika, when Soviet scholars revealed in 1989 that Joseph Stalin had indeed ordered the massacre and in 1990, Mikhail Gorbachev officially recognised that the NKVD had executed the Poles, personally initiated by Beria and Merkulov. Much later, the Russian Duma approved a declaration in November 2010, blaming Stalin and other Soviet officials for ordering the massacre.