Such is the gravity of Russian president Vladimir Putin’s aggression against Ukraine that an international prosecutor should be appointed and a tribunal established to bring him and his collaborators to trial for their crimes.

Ukrainian president Volodymyr Zelensky and his foreign minister pleaded with the world this week for action based on the principles that guided the Allied countries in 1942 when they gathered in London to promulgate a decree entitled, “Punishment for War Crimes”. Just as the Second World War Allies vowed 80 years ago to indict Nazi war criminals, Ukrainian foreign minister Dmytro Kuleba is asking that Putin be brought to justice for the same kind of “crimes against peace”.

With his armies surrounding Ukraine’s major cities and firing indiscriminately on inhabitants, the catalogue of Putin’s atrocities grows: the killing of civilians, including children; repeated breaches of ceasefires and humanitarian corridors; nuclear brinkmanship; and threatening the public execution of civilians who do not bend to his diktats.

Yet a gap in international law prevents the International Criminal Court from investigating and prosecuting Russia for the most basic war crime of all: aggression, even though aggression is recognised as a crime in many countries’ own national laws — including those of Russia and Ukraine.

The ICC has powers to investigate crimes against humanity, war crimes and genocide; but Russia never signed on to an amendment establishing aggression as a crime and therefore is not bound by it. And that stymies the international community’s fastest and simplest path to holding Putin accountable for waging an illegal war.

So while the United Nations General Assembly voted overwhelmingly to recognise that the Ukraine invasion lacks any legal justification and that it constitutes the international offence of aggression in violation of the UN Charter, there is no obvious legal route to charge Putin for that overarching offence. The European Court of Human Rights can investigate violations of civil liberties, and the UN’s International Court of Justice can examine Ukraine’s complaint against Russia, but the aggressor cannot be arraigned as was done when setting up the international military tribunals that eventually led to the Nuremberg trials.

In the spirit of the 1942 agreement, I and more than 100 former and present European leaders and international lawyers are urging the United States to join us in supporting a special tribunal to try Putin for his crimes of aggression. There is an American precedent that might encourage President Joe Biden to seize the moment. In 1993 and 1994, President Bill Clinton pushed for and succeeded in establishing special tribunals for dealing with war crimes in Yugoslavia and Rwanda.

Pressing for such a tribunal is, as the Ukrainians have told us, important to boosting the morale of a beleaguered but resilient people. Although lives are being destroyed, Ukrainian unity seems indestructible. Hearts may be broken every hour of the day, but their resolve has proved unbreakable. Buildings are being razed to the ground, but the spirit of the people is lifting up the world. We, in turn, must lift them up, not just in word but in deed.

A tribunal would also send a message to Putin and his inner circle that their brutal criminality will not escape trial and punishment. It would sow fear among Putin’s inner circle, if not Putin himself. Just as those complicit with Nazi crimes started to peel away from their leader and seek private deals, so, too, may Putin’s collaborators begin to co-operate with the forces of justice.

The way forward is for a group of governments to support Ukraine in setting up such a tribunal. As a first step, we must record the crimes of continued aggression as they happen. An office should be staffed in The Hague — home to the ICC — to collect evidence and investigate how best to proceed with organising this route to justice. A small staff should work closely with the ICC and, given that the crime of aggression is already well understood, move with speed. By summer a tribunal could be in place and Putin indicted.

“Grand strategy looks beyond the war to the subsequent peace,” wrote the historian Basil Liddell Hart. So even as Ukraine is ravaged by bombs and bullets, it is right to think now of what comes after and how best to achieve reconciliation, peace and justice for Ukrainians.

Not since 1942 have the words issued in the decree of the Allies been more apposite. Our aim must be, they said, “to secure the punishment, through the channel of organised justice, of those guilty or responsible” and “to satisfy the sense of justice of the civilised world”.

The people of Ukraine deserve nothing less.

• Gordon Brown was Prime Minister of Britain from 2007 to 2010 and is at present the United Nations Special Envoy on Global Education. This opinion first appeared on Bloomberg