Russia’s decision to wage a war in Ukraine, and to continue that war even after the International Court of Justice in The Hague has ordered a halt, is manifestly illegal. Under international law, Russia’s invasion has no justification: It is not self-defense, has not been authorized by the UN Security Council, and serves no humanitarian purpose. Quite the opposite. It is a war that gives rise to individual criminal responsibility for those who are prosecuting it: Vladimir Putin, his inner circle, the financiers, and all others who have contributed materially to the direction taken.

This crime entered the lexicon of international law at the Nuremberg trials, introduced by a Soviet jurist, Aron Trainin, who described “crimes against peace.” The Nuremberg judges called it “the universal crime,” from which all others flowed. Today it is called the crime of aggression. It can be prosecuted under the national law of many countries, including Russia and Ukraine. It cannot, however, currently be addressed by the International Criminal Court (ICC), whose jurisdiction in relation to Ukraine covers only war crimes (the manner in which the war is conducted, including the targeting of civilians); crimes against humanity (the systematic destruction of civilians); and genocide (the intentional destruction of groups). No other international tribunal has the power to investigate the crime of aggression in Ukraine.

This is why a month ago I proposed that this serious gap be filled. After all, the crime of aggression is the only one that can target with certainty those who are most responsible for the horrors being heaped on millions of human beings. Perhaps it can be proved that Putin and those in his inner circle are personally responsible for the war crimes and crimes against humanity that appear to have been committed in Bucha, Mariupol, and across Ukraine, which are being investigated by national prosecutors and the ICC prosecutor. But it is far from certain that the evidence will lead all the way to the top. There is a real possibility that, some years down the line, the ICC will find itself prosecuting individuals alleged to have done terrible things, but not those individuals who have done the most terrible things.

On April 5, President Volodymyr Zelensky called for the creation of a special international criminal tribunal, to which should be delegated powers under the laws of Ukraine to investigate those who have waged this terrible war. The call is supported by dozens of former prime ministers and presidents, and a petition sponsored by the nonprofit organization Avaaz has attracted nearly 2 million supporters. Several European governments are now considering how such a tribunal could be created. The idea offers hope and solidarity to Ukraine and its people, a boost to morale. It would further delegitimize Putin. It might create an incentive for some within his circle to peel off, as some senior Nazis did in the spring of 1945. It would also offer leverage in future negotiations. It would signal that values and principles matter, that impunity at the top is not an option.

The Biden administration is hesitant to unleash a process that could actually ensnare Putin and his team—not because of any legal or operational concerns, or as a matter of principle, but because, like Britain and France, it worries about the precedent. Russia today, perhaps the US tomorrow. Similar concerns have long kept the US from joining the ICC—and that was before the shadow of Iraq, also an illegal war, lingered over such questions.

The international rule of law is a fragile creature. Those who have a semidetached relationship to it, including the United States, are perhaps not best placed to invoke it for the crimes of others. Yet the law has a force and authority distinct from economic, military, or other instruments of power. Our failure to address this crime of aggression, especially when perpetrated on so horrific a scale, lets Putin off the hook and undermines the very idea of accountability. The crime of aggression gave rise to all the other crimes that have followed. Those at the top of the chain of command must be pursued.