As a scholar of modern German history, I've been working on a study of anti-Semitism in Germany and the United States in the late nineteenth and early twentieth centuries. What I saw unfold over the weekend in Charlottesville, Virginia, and then at Bedminster, New Jersey, gave me the horrible, sinking feeling that my book is going to need a new chapter.

On Saturday 12 August 2017, thousands of young Americans marched through the streets of Charlottesville, chanting hate-filled slogans like “blood and soil” and “Jews will not replace us,” and carrying the swastika flag. They clashed with protesters and caused dozens of injuries. A car ploughed into a crowd of people protesting the white supremacist demonstration, killing one person and injuring many more.

Later that day, President Donald Trump issued a statement: “We condemn in the strongest possible terms this egregious display of hatred, bigotry and violence on many sides, on many sides. It has been going on for a long time in our country… not Donald Trump, not Barack Obama. It has been going on for a long, long time. It has no place in America.”

The “hatred, bigotry and violence” he said, came from “many sides” (a point he apparently felt he needed to stress). He did not mention the fact that one side was carrying swastika flags, the flag of Adolf Hitler’s National Socialist Party, the flag of Nazi Germany. He did not specifically condemn those who carried that flag. They were, according to the president, all equally responsible: those who marched under the Nazi banner, and those who opposed them. All equal. Nazis and anti-Nazis.

Violence on the streets of Charlottesville Show all 9 1 /9 Violence on the streets of Charlottesville Violence on the streets of Charlottesville Protesters clash and several are injured White nationalist demonstrators clash with counter demonstrators at the entrance to Lee Park in Charlottesville, Virginia. A state of emergency is declared, August 12 2017 Violence on the streets of Charlottesville Trump supporters at the protest A white nationalist demonstrator walks into Lee Park in Charlottesville, Va., Saturday, Aug. 12, 2017. Hundreds of people chanted, threw punches, hurled water bottles and unleashed chemical sprays on each other Saturday after violence erupted at a white nationalist rally in Virginia. AP Photo Violence on the streets of Charlottesville State police stand ready in riot gear Virginia State Police cordon off an area around the site where a car ran into a group of protesters after a white nationalist rally in Charlottesville, Virginia AP Photo Violence on the streets of Charlottesville Militia armed with assault rifles White nationalists, neo-Nazis and members of the 'alt-right' with body armor and combat weapons evacuate comrades who were pepper sprayed after the 'Unite the Right' rally was declared a unlawful gathering by Virginia State Police. Militia members marched through the city earlier in the day, armed with assault rifles. Getty Images Violence on the streets of Charlottesville Statue of Confederate General Robert E Lee The statue of Confederate General Robert E. Lee stands behind a crowd of hundreds of white nationalists, neo-Nazis and members of the 'alt-right' during the 'Unite the Right' rally 12 August 2017 in Charlottesville, Virginia. They are protesting the removal of the statue from Emancipation Park in the city. Getty Images Violence on the streets of Charlottesville Racial tensions sparked the violence White nationalists, neo-Nazis and members of the 'alt-right' exchange insults with counter-protesters as they attempt to guard the entrance to Lee Park during the 'Unite the Right' rally Getty Violence on the streets of Charlottesville A car plows through protesters A vehicle drives into a group of protesters demonstrating against a white nationalist rally in Charlottesville, Virginia. The incident resulted in multiple injuries, some life-threatening, and one death. AP Photo Violence on the streets of Charlottesville Rescue personnel help injured people after a car ran into a large group of protesters after an white nationalist rally in Charlottesville, Virginia AP Photo Violence on the streets of Charlottesville President Donald Trump speaks about the ongoing situation in Charlottesville, Virginia from his golf club in Bedminster, New Jersey. He spoke about "loyalty" and "healing wounds" left by decades of racism.

But how is that possible? How can it be that in 2017, the President of the United States, a country that fought Hitler’s Germany and sacrificed hundreds of thousands of its young men in order to ensure its ultimate defeat, could not or would not bring himself to condemn Americans who marched under the flag of the Third Reich?

What does it mean to march under the swastika flag? What does the swastika flag symbolise? What did it mean to the people who hoisted it in Germany – the people who inspired the Americans who marched this weekend in Charlottesville?

Those who inspired the marchers in Charlottesville marched through the streets of Germany, provoking violence, and singing “when Jewish blood spurts from the knife.”

Those who inspired the marchers destroyed democracy and eliminated all civil liberties in Germany.

Those who inspired the marchers demonised Jewish citizens, physically assaulted them, removed them from all aspects of public life, stripped them of their rights, their property, their very ability to survive in the only country they had ever called home.

Those who inspired the marchers carried out the biggest pogrom in modern German history, destroying 267 synagogues, vandalising Jewish businesses, attacking Jews in their homes, and killing hundreds, all in a single night in November 1938.

They demonised and physically attacked political opponents, homosexuals, Roma and Sinti, the handicapped, and any others they considered outside the boundaries of the German racial community.

They murdered more than 70,000 men, women, and children – German citizens! – who had been diagnosed with mental and physical disabilities in just two years between 1939 and 1941.

They started the most destructive war in the history of the world, causing the deaths of tens of millions of people, mostly innocent civilians.

They murdered more than 33,000 Jews in just two days at Babi Yar, outside Kiev, Ukraine, in 1941.

They shot one million unarmed Jewish civilians – men, women, and children – across Eastern Europe in just the last six months of 1941.

They murdered close to three million Jews in the gas chambers of Chelmno, Treblinka, Sobibor, Belzec, Majdanek and Auschwitz.

They enslaved millions of people – Jews and non-Jews – from across Europe to work for their war of conquest.

They fought to destroy the most basic values that America has claimed to stand for over more than two centuries: the fundamental dignity and equality of all people.

The world is a complicated place. There are rarely simple, black and white answers to the problems that confront us. But sometimes, every once in a while, there are. And this is one such moment. If the President of the United States cannot condemn individuals who march under the flag of Adolf Hitler and Nazi Germany, how can he possibly claim to represent America, its values, and all of its citizens? In perhaps the easiest test of his young presidency, Donald Trump has failed, and failed miserably.

Jeremy Corbyn slams Donald Trump over Charlottesville response

It isn’t often that historians get to see their work gain such relevance in the present. And for those of us who study the history of hatred, bigotry, and the evils of Nazi Germany, the prospect of such relevance is most uncomfortable. If my work has taught me anything, it’s the importance of keeping the boundaries of one’s moral universe as wide as possible. In the early twentieth century, too many Germans pushed too many others beyond the boundaries of their moral universe – beyond the borders of the German racial community – where their fate was at best no longer of any concern to them, at worst, they represented an existential threat.

When that happens, the horrors committed under the swastika flag become possible. How safe are we today? How extensive are the boundaries of our own moral universe – each and every one of us? Those who marched in Charlottesville under Hitler’s flag and the President who chose not to condemn them revealed the boundaries of their moral universe to be sadly and frighteningly small. The flag that flew on that horrible day – with that symbol of ultimate evil at its heart – should remind us all just where such a limited sense of fellow feeling can lead.