Complex sentence: The first and best known set of these trials, those of the major war criminals, tried before the IMT, were described as "the greatest trial in history" by Norman Birkett, one of the British judges who presided over them.
Split into simpler sentences: The first and best known set of these trials were those of the major war criminals before the International Military Tribunal (IMT). They were described as "the greatest trial in history" by Norman Birkett, one of the British judges who presided over them.