After the end of the war, Lahousen voluntarily testified against Hermann Göring and 21 other defendants at the Nuremberg war crimes trials in 1945–1946. Lahousen was the first witness for the prosecution, his prominence due to being the sole survivor of the 'Abwehr resistance'. Among other things, he gave evidence about the murder of hundreds of thousands of Soviet prisoners of war and the Einsatzgruppen death squads, who murdered more than a million Jews in the conquered areas of the Soviet Union, Poland and Ukraine.