The Germans understood the political potential of what they had found. Twenty-two thousand Polish officers, intellectuals, civil servants, priests, and policemen had vanished into Soviet hands during the 1939 partition of Poland, and now the bodies of thousands of them were emerging from the soil of an ally of the Soviets' adversary. Berlin announced the discovery in April 1943 and called for an international commission to confirm Soviet guilt. The selected experts came from eleven European countries, most of them German allies or occupied nations: Romania, the Netherlands, Hungary, Italy, Croatia, Bulgaria, Slovakia, Finland, Denmark, Belgium. One member, the Genevan professor François Naville, came from neutral Switzerland; his presence was meant to lend the proceedings a credibility the German invitation alone could not provide.

The forensic conclusions were unambiguous. The state of decomposition, the documents recovered from the corpses, the manner of execution, the type of ammunition recovered: every line of evidence pointed to the spring of 1940, when this forest lay deep within Soviet-controlled territory. The commission signed its report on April 30, 1943. The Polish government-in-exile in London asked the Red Cross to corroborate the findings. Stalin used that request as his pretext to break diplomatic relations with the Polish exiles, a rupture that would shape Poland's postwar fate. The Germans had won their wedge between the Allies. The Soviets responded with a counter-commission of their own: the Burdenko Commission, which in 1944 declared the killings a German atrocity and produced documents to support the lie.

The signatories paid for what they had done. After the war, those who lived in countries that fell behind the Iron Curtain were pressured, jailed, or driven into exile to retract their findings. The Croatian Eduard Miloslavić fled to the United States. The Czech and Bulgarian members were imprisoned. The Slovak pathologist Andrej Žarnov, whose pen name marked him as one of his country's important poets, escaped to the West rather than recant. Naville, in neutral Geneva, defended the report for the rest of his life and refused every Soviet pressure to withdraw it. Across the West, Cold War politics buried what these scientists had documented. The United States knew the truth by 1944 and chose not to publicize it. Britain knew. The truth waited.

In April 1990, Mikhail Gorbachev formally acknowledged that the NKVD had carried out the killings. Soviet archives released over the following years confirmed that Lavrentiy Beria had recommended the executions in March 1940 and that Stalin had signed the order. The total number of those killed across Katyn, Mednoye, Kharkiv, and other sites came to 21,857 Poles. Among them were fourteen generals and an admiral, named officer by officer in the recovered documents. In 2010, the Russian State Duma issued a formal declaration confirming Stalin's personal responsibility. In the years since, that admission has eroded under the current Russian state's renewed claims about wartime history. The forensic evidence the 1943 commission documented, however, has only been confirmed and reconfirmed by every honest investigation since.

The story of the Katyn Commission is a story about what scientists do when politics surrounds their work. The men who walked into that forest were used by Nazi propaganda; they knew it. They did the work anyway. Their report was technically accurate and morally clear, and it took half a century for the world to formally agree with what they wrote in three weeks. The forest at Katyn is now a cemetery. Their findings sit at its center.