In 1943, informed by Poles living in London about the massacre in the woods at Katyn of 15,000 Polish servicemen by Britain's ally, the Soviet Union, Potocki published what he considered to be his most important piece of writing, his "Katyn Manifesto". The British government had been keen to keep the atrocity quiet, which the Soviet Union blamed on Nazi Germany. Potocki was arrested by Special Branch and imprisoned. Later, he was sent to an agricultural camp in Northumberland. This manifesto was the only acknowledgement of the atrocity in English. The full truth of the Katyn massacre was not to emerge for another 50 years. His arrest and conviction was reported as related to the blackout regulations as he was showing a light, and he was reported as saying in court "Heil Hitler". In 1945 Potocki criticized the Nuremberg Trials describing them as 'a revolting travesty of justice'.