In the spring of 1943 the Nazi Government in Germany announced that a mass grave had been found in Katyn Forest near Smolensk in the Soviet Union. Over 1,700 bodies were discovered and the Germans claimed that men were Polish soldiers who had been murdered by being shot in the head. It was suggested that the men had been killed by the NKVD.

A Polish government-in-exile in London under the leadership of Wladyslaw Raczkiewicz, Wladyslaw Sikorski and Stanislaw Mikolajczyk, demanded an investigation of the deaths by the Red Cross. Joseph Stalin refused claiming that the Poles was a victim of Nazi propaganda. When they continued to complain Stalin decided to break off relations with the Polish government.

After the Second World War it was discovered that over 10,000 Polish soldiers being held in three prison camps near Smolensk were unaccounted for.

In 1989, Mikhail Gorbachev revealled to the world that in March 1940, Joseph Stalin had given the orders for the execution of 25,700 Polish soldiers in Soviet prison camps. He also admitted that two other mass graves had been found in the Katyn area.