The legacies of Tokyo Trial have been overlooked and questioned partly because prosecuting aggression was allegedly a violation of the principle of legality. This essay argues that the trial should not be overlooked for this reason because the legality debate at the trial provides insights into the interplay between the principle of legality and sources of international criminal law. Besides the majority judgment, some minority opinions could shed light on the nature of the Tokyo Charter by distinguishing between jurisdiction and applicable law and link the issue to the legality challenge. Although the Tokyo Charter was formally different from the Nuremberg Charter, both of them are substantive in nature so that the tribunals were allowed not to address the legality challenge. In addition, prosecuting aggression was arguably not a violation of the principle of legality because this principle, at that time, did not bind ex post facto legislation against international crimes committed during World War II.

• Xiao Mao(The Center for Tokyo Trial Studies of Shanghai Jiao Tong University)