It was called the biggest murder trial in history. Twenty-two members of the Einsatzgruppen, Nazi extermination squads responsible for the deaths of more than a million Jews, and many thousands of Gypsies, partisans and others, were tried and convicted at Nuremberg. The chief prosecutor for the US in the case was former army sergeant Benjamin Ferencz. It was his first trial, and he was 27 at the time.

This year marks the 70th anniversary of that case and the global postwar order is unravelling at a rapid pace. From the UK’s vote to leave the EU, to the election of Donald Trump; from the UN’s stand-off with Syria and its allies for atrocities committed in its civil war to a colossal refugee crisis, peace and security have not seemed this unstable since the second world war. But this doesn’t dampen Ferencz’s mood.

“I’m always doing fantastic,” he tells me with certain mirth, when I call him at his home in New Rochelle, New York. “You know why? Because I’m 97 years old and I’m aware of the alternatives.”

There is nobody alive in the world with Ferencz’s perspective. As the last surviving prosecutor of the Nuremberg trials, he has witnessed more in his lifetime than most. He was born into a Jewish family in Transylvania in 1920 and they moved to New York when he was 10 months old. Growing up in a neighbourhood riddled with crime, he was a studious adolescent keen to prove his worth. He did so by winning a scholarship to Harvard law school. After graduating he joined an anti-aircraft artillery battalion of the US army and “received five battle stars from the Pentagon for having not been killed in every major battle in Europe”. He landed on the beaches of Normandy during the D-day offensive; broke through the German defences on the Maginot and Siegfried lines; crossed the Rhine at Remagen; and took part in the Battle of the Bulge at Bastogne.

In 1945, as Nazi atrocities were uncovered, Ferencz was transferred to the headquarters of General Patton’s third army, tasked with setting up a new war-crimes branch. He was present at, or arrived soon after, the liberation of concentration camps including Buchenwald, Mauthausen, Flossenbürg and Ebensee, scouring the barbarous scenes for evidence of Nazi wrongdoing to present at trial. The most significants items he collected, he says, were the death registries, kept as meticulously by the Germans as hospital birth certificates.

“There were 3,000 men who, for two years, murdered people, including children and infants,” Ferencz recalls with a memory that defies his years. “One shot at a time, or, as one of my lead defendants, who killed 90,000, instructed his troops: ‘If the mother is holding an infant to her breast, don’t shoot the mother, shoot the infant because the bullet will go through both of them, and you’ll save ammunition.’”

The defendants, he explains, were picked on the basis of their rank and education. “But then the decisive absurdity: why only 22? Well, there were only 22 seats in the dock. It was ridiculous, but it was symbolic. We were trying to show people how horrible it is if you take a leader who’s very charismatic, and unquestionably follow him, even to murdering little children. These were educated people; one was a father of five children. They were not all wild beasts with horns.”

Ferencz at the Einsatzgruppen trial at Nuremberg. Photograph: Alamy Stock Photo

It was far from a painless job, and what Ferencz saw in the concentration camps haunts him to this day. “It’s unimaginable. Bodies lying around; you can’t tell if they’re dead or alive, pleading with their eyes for help. Waving their hand and you see they’re alive, in rags; rats, dysentery, diarrhoea, every disease in the camps. It was an experience indescribable because of its horror. It was as if I had peered into hell. That’s why I’m still fighting, to prevent that from happening again.”

This fight includes convincing the world that countries are stronger and safer together than they are apart, at a time when national interests are increasingly being emphasised over international laws.

“My general reasoning is that the world is a small planet,” Ferencz says. “We must share the resources on this planet, so that everyone can live in peace and human dignity, and it can be done. The recognition that we have to move as a unit gave us the EU, it gave us the US, 50 states with very differing opinions. Most wars are fought against another group, the ‘other’. When you are a part of the other, you’re less inclined to attack it.

His three pieces of advice to young people would be: 'One, never give up. Two, never give up. Three, never give up'

“So, the movement towards unification was itself a great accomplishment. Will there be economic problems? Of course. If one is rich and the other one is poor, the rich doesn’t want to give it up to the poor. But Brexit is a mistake, and in time they will look back at this and think: ‘Oh my God, how could we have been so backward?’ Today, some people are fleeing for their lives, other people are saying: ‘Not in my backyard, take them someplace else, they speak a different language, they don’t want to work’ – all that junk. But you wouldn’t treat your family that way, I hope. We’re all brothers and sisters, and Great Britain, with its large colonies, should certainly recognise that.”

The European convention on human rights, he adds, is another expression of hope, similar to the universal declaration of human rights, drawn up following the war. “Is Theresa May doing the right thing in backing away from the ECHR? I don’t think so; it may be good for the next election, but it’s shortsighted. Unfortunately, the public isn’t interested in things like distant courts in distant places, argued by lawyers who are quibbling about obscure phrases in complex law.”

On sovereignty and nationalism, Ferencz says the popularity of figures such as Trump, Marine Le Pen and Nigel Farage was never beyond the realms of possibility. “People always have differences of opinion, and when the shoe hurts the foot they’ve got to kick somebody, and they’re not always rational. A man like Mr Trump doesn’t fit the pattern of trained politician or international realist, optimist or idealist, but almost half the people voted for him.”

The way to tackle politicians who peddle hate, he continues, is to try to change the opinions you don’t agree with through compassion, compromise and courage. “You begin at the earliest level. When little Johnny is playing baseball with little Tommy and he doesn’t like what Tommy does, you teach him he doesn’t hit him with a bat, he talks to him and tries to settle it.

“My slogan has always been ‘law not war’. You would save billions every day and be able to take care of refugees who don’t have a home, students who can’t afford tuition, the poor and the elderly. Think of all the money we are wasting on preserving the outdated nuclear weapons, which nobody knows what to do with and which are obsolete.”

He remains optimistic about society’s inevitable advancement, however. “Fundamental things such as colonialism and slavery, the rights of women, the emancipation of sex, landing on the moon, these were inconceivable not long ago. But miracles can be performed.”

Since the second world war, Ferencz has stayed busy, leading efforts to return property to Holocaust survivors, participating in reparations negotiations between Israel and West Germany. In 1970, as the US was embroiled in Vietnam, he decided to withdraw from private practice altogether and devote himself to promoting peace. He wrote several books outlining his ideas for an international legal body, which became fundamental in the establishment of the International Criminal Court in The Hague. When the ICC came to their first case, against Congolese warlord Thomas Lubanga Dyilo in Uganda, they invited Ferencz to make the closing remarks for the prosecution.

But the US, China and Israel have refused membership, and at the end of last year, Russia joined African states including South Africa and Gambia in formally withdrawing its signature from the Rome statute, just one day after the court published a report classifying the Russian annexation of Crimea as an occupation. Even the UK has limited the court’s funding. These are heavy blows to efforts to establish a global legal order for pursuing genocide, war crimes and crimes against humanity, and it is now feared that the tide of nationalist sentiment is threatening to undermine the project altogether.

A force for good ... Benjamin Ferencz. Photograph: Brooks Kraft/Getty Images

“The Russians withdrawing is a sign of protest, saying: ‘You can’t push us around.’ It’s a dangerous game they’re playing, and it’s unfortunate.” The US’s refusal to commit, Ferencz adds, “would be laughable if it wasn’t so tragic ... the most powerful nations of the world are not yet ready to surrender what they perceive as a sovereign right to use whatever means they perceive to be necessary in order to protect their own interests as they see them.”

The ICC’s biggest problem, then, is that it lacks any meaningful enforcement powers. “You need laws in order to define what’s permissible and what’s not permissible, you need courts where people can be held accountable if they violate the laws, and you need a system of effective enforcement. Those are the three legs on which civilisation stands. But we only have the two legs, and they’re both a little bit wobbly; the third enforcement leg doesn’t even exist.”

For his work and achievements, Ferencz has received many awards, including the medal of freedom from Harvard in 2014. He continues to be a force for good, last autumn donating $1m (£800,000) to the Holocaust museum’s genocide prevention centre, a gift he has pledged to repeat annually for up to 10 years. “I don’t care about glory, I don’t care about legacy, I don’t care about money, I’d give it all away,” he says. “I came into the world as a pauper, I lived most of my life in poverty, and now I’m giving it all back.”

I ask him if he was nervous, all those years ago, as he faced the court at Nuremberg. “No, I was confident. Was I sad? No, I was determined. Was I worried? No, I was angry.”

If he had to give three pieces of advice to young people, they would be: “One, never give up. Two, never give up. Three, never give up.”

These days, he spends much of his time in Florida with his wife Gertrude; they have four children, all of whom have retired. “My wife is a few years older than me, she couldn’t take the cold of New York at this time of year. We’ve been 70 years wed without a quarrel. So I’ve got to get back to Florida and continue to take care of her.”

There’s a pause, before he adds: “There are other things to do besides saving the world, you know, my dear.”