The Nuremberg trials of Nazis that took place in 1945. Not many years later with confused or otherwise altered memories.
Even the Germans who testified said it happened as we know it did today. They didn't deny the Holocaust happened, they claimed to be following Government orders.
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http://www.ushmm.org/wlc/en/article....uleId=10007271
"Eyewitness testimony from both perpetrators and survivors laid the foundation for much of what we know about the Holocaust, including details of the Auschwitz death machinery, atrocities committed by the Einsatzgruppen and other SS and police units, the destruction of the Warsaw ghetto, and the original statistical estimate of six million murdered Jews. Many people directly involved in the killing program died before the end of the war, but the Allies interrogated many of those who were still alive in preparation for the trial. None of the perpetrators denied the Holocaust. Most just tried to deflect their responsibility for the killings.
Three key perpetrators gave evidence directly related to the Holocaust: Hermann Göring, the highest official of the Nazi state tried at Nuremberg, testified openly and frankly about the persecution of German Jews from the rise of the Nazi party to power in 1933 until the outbreak of war in 1939; Otto Ohlendorf testified directly about his unit, Einsatzgruppe D, killing 90,000 Jews in the southern Ukraine in 1941; and the commandant of Auschwitz, Rudolf Hoess, testified frankly about the gassing of more than a million Jews at the Auschwitz-Birkenau killing center during the war. All three claimed that they carried out the legitimate orders of the state.
While the testimony of perpetrators is often chilling in its frankness about the killing program, testimony from survivors, then and today, is often the best antidote to Holocaust denial. Holocaust survivors directly experienced Nazi genocidal policies. Their testimony is personal, immediate, and, for this reason, compelling."