NUREMBERG, Germany -- Eighty years ago this October, judges in Courtroom 600 in the southern German city of Nuremberg handed down a judgment that established a fundamental principle of international criminal justice: Starting a war of aggression was not merely an international crime, but "the supreme international crime." "To initiate a war of aggression ... is not only an international crime; it is the supreme international crime differing only from other war crimes in that it contains within itself the accumulated evil of the whole," the judgment said.

Yet eight decades later, the international community has still not fully resolved how to punish that crime. Russia's invasion of Ukraine has exposed the limits of the system built in the aftermath of World War II, while U.S. President Donald Trump's sanctions against the International Criminal Court have put pressure on the institution intended to make international criminal justice permanent.

For Japan, the moment poses a particular question. Tokyo has long been a major financial and personnel supporter of the ICC, but it has stopped short of accepting amendments that expanded the court's jurisdiction over the crime of aggression.

That tension was hanging over the Nuremberg Forum 2026, an event organized by the International Nuremberg Principles Academy and held Oct. 7-9 in Courtroom 600 to mark the 80th anniversary of the Nuremberg judgment.

"Russia's full-scale invasion made it impossible to maintain that aggression is essentially a matter of the past," Claus Kress, a professor at the University of Cologne and special adviser on the crime of aggression to the Office of the Prosecutor of the ICC, said in his keynote address on the opening day.

The challenge extends beyond the crime of aggression. With the Trump administration targeting the ICC with sanctions participants warned that the institution itself must first survive.

"First thing is, we need to make sure that the ICC is still in place. I think that is the foremost and first measure," said Franz Xaver Perrez, head of the Directorate of International Law at Switzerland's Federal Department of Foreign Affairs.

For Japan, Nuremberg has a close parallel in the International Military Tribunal for the Far East, better known as the Tokyo Trial. The tribunal remains familiar to Japanese people and continues to attract criticism as "victors' justice." But Kress argues Tokyo was indispensable to the development of international criminal justice.

"Even though the Tokyo Trial is more complex, it is an essential part of the evolution of international criminal law and international criminal justice," he said. A view of the defendents' dock during the Nuremberg Trials in Nuremberg, Germany.

The principles established at Nuremberg and Tokyo were carried forward through subsequent special tribunals and ultimately into the permanent ICC. The court was conceived as a more neutral and permanent institution, taking into account criticism that the postwar tribunals amounted to "victors' justice" and applied law retroactively.

Japan, despite complicated domestic attitudes toward the Tokyo Trial, has supported the development of international criminal justice and become one of the ICC's largest supporters, providing both financial and personnel support. e of Nuremberg's central legacies, however, Tokyo has taken a more cautious approach.

Although the Nuremberg judgment described aggressive war as "the supreme international crime," aggression long remained a gap in the international criminal justice system. The crime was included in the Rome Statute establishing the ICC, but agreement on its precise definition and the court's jurisdiction was deferred because of its implications for national sovereignty and security.

After lengthy negotiations, the ICC gained the ability to exercise jurisdiction over the crime of aggression in 2018, 17 years after the court was established.

Japan participated in those negotiations but has not accepted the Kampala amendments to the Rome Statute that activated the court's jurisdiction over aggression, citing remaining uncertainties over the definition of the crime and the scope of the court's jurisdiction.

David Donat Cattin, senior fellow at the Montreal Institute for Global Security, criticized Japan's position.

"I think this is very embarrassing for a nation state that participated with Italy and Germany in World War II. How many Japanese citizens are aware of the fact that their government is not supporting such an important amendment to the Rome Statute," he said.

Japan decided to join the Rome Statute in 2007 under the first government of Prime Minister Shinzo Abe. Cattin said Japan's current leaders should show the same leadership Abe demonstrated when Japan joined the ICC. Kress also urged Japan to take a more assertive role.

"I am convinced that Japan has nothing to fear. I hope that Japan and Germany, as important middle powers, will invest more in protecting the system, not only economically but also through political courage and determination," he said.

Even after the ICC gained jurisdiction over aggression, significant restrictions remain. Its jurisdiction does not extend to Russia's aggression against Ukraine, one reason efforts are underway to establish a special tribunal for Ukraine.

For Kress, the need for such a tribunal demonstrates how far the international system still has to go.

"It shows that we have not really been able to pass beyond Nuremberg and Tokyo. Legally speaking, it is, in a sense, a repetition of the Nuremberg or Tokyo moment," he said.

ICC member states remain divided over proposals to further strengthen the court's jurisdiction over the crime of aggression, but are aiming to reach a conclusion by 2029. Kress argues that, until then, countries need to preserve and strengthen the ICC rather than yield to pressure from the Trump administration.

Courtroom 600, where the forum took place, was still being used as a working courtroom until only a few years ago. The debate over aggression that began there is also still alive 80 years later.

With the ICC itself under pressure, Japan faces a question that goes beyond continuing to support the court financially and with personnel. It must also decide what role it is prepared to play in strengthening the international system's ability to hold those who launch wars of aggression accountable.