It was no accident that the atomic bomb and the Holocaust came about at the same time. When the Nazis came to power in Germany in 1933, neither was on anybody's agenda. Nor was the intention to systematically kill Jews - that would come later. Instead, the top item on the Nazi master-plan was to rid German culture of perceived pollution. Artists were to be asked to sign an oath of loyalty. But the biggest impact came in science.
Paradoxically, German science was already the envy of the world. Loud voices who wanted to safeguard this position complained about contamination by incomprehensible new ideas such as relativity and quantum theory. These were the result, said Nobel Prize winner and German physicist Philipp Lenard, of a "massive infiltration of the Jews into universities… The most obvious example of this damaging influence was provided by Herr Einstein." The Jews in general and Albert Einstein in particular were to become the target of an intellectual purge.
When a new political party comes to power, its first enactments are highly symbolic. The Gesetz zur Wiederherstellung des Berufsbeamtentums - the cynically named Law for the Restoration of the Civil Service - was published on April 7 1933. To "simplify the administration", officials who had not undergone "the usual training" or were not "suitable" would lose their jobs. Another paragraph explicitly targeted those of "non-Aryan descent", without saying exactly what this meant.
This was clarified a few days later with the first Edict for the Implementation of the Law for the Restoration of the Civil Service. This targeted anyone who had non-Aryan, "particularly Jewish", parents or grandparents. Having just one single Jewish grandparent was enough to lose one's job.
At the beginning of the 20th century, many German Jewish infants were baptised, a move seen as expedient rather than spiritually fulfilling. But such pragmatism was now deemed irrelevant; parents and all grandparents would also need to have been baptised themselves. Not many had. It had been too early. And so, under the Civil Service Law, thousands of university teachers, together with doctors, lawyers and other professionals, were sacked. Some found more modest jobs, some retired, some left the country.
With the Civil Service Law, Germany was throwing away its hard-won scientific supremacy. When warned of this, Hitler retorted: "If the dismissal of [Jews] means the end of German science, then we will do without science for a few years."
Why did Jews have so much influence on German science? They had a long tradition of study, as evidenced in the many generations of talmudic students. But assimilated German Jews began to look instead to new intellectual horizons, where a radiant role-model appeared in the shape of Albert Einstein.
Einstein changed our view of the world. His revolutionary theories made him not only the most famous scientist in the world, but the most famous scientist the world had ever known. Einstein became an icon for ambitious young students - and a ready target for critics in high places. Aware of this polarised reputation, he left Germany in 1932 for a new job in the United States with an annual salary of $15,000 - five times more than he had requested.
The following spring, Einstein briefly returned to Europe, renting a house on the Belgian coast. He did not dare show up in Berlin. His house there had been broken into, and his bank account blocked. From Belgium he resigned from the Prussian Academy of Science, which had objected to his "smear campaign" and so had "no reasons for regretting this departure".
By this time, the Civil Service Law was in force, and many intellectuals were trying to emigrate. Before returning to the US, Einstein stopped off in London, where he addressed a rally at the Albert Hall to raise money for academic refugees who could not command salaries of $15,000.
A contemporary, ironic development flowed from the fact that a remarkably talented Jewish generation had grown up around the turn of the century in Budapest, where Jews made up about 20 per cent of the population. But, in the trauma following the fall of the Austro-Hungarian Empire in 1918 and the emergence of the modern state of Hungary, many Jews emigrated. The scientists of this generation went to Germany, which was back then more hospitable to Jews than Hungary was.
One was Leo Szilard, a talented scientist one of whose remarkable abilities was to foresee crises before they happened. He also knew how to wield influence, and carefully collected friendships with influential individuals. In Berlin, Szilard met Einstein. Between them, they registered many ingenious patents, including a new type of silent refrigerator. But the Einstein-Szilard link would go on to be more significant than this.
Like Einstein, Szilard anticipated the Civil Service Law. In April 1933, he took a train to Vienna, his baggage stuffed with money to bankroll his future. His timing was impeccable: the very next day, the same train was packed with Jewish refugees who were stopped at the border to check that their exit papers were in order.
Staying at the same Vienna hotel as Szilard was the British economist William Beveridge, then director of the London School of Economics, and the man who would later provide guidelines for the introduction of the welfare state in Britain. Beveridge had influence and, at Szilard's urging, helped organise a scheme to assist refugee German academics. Thus was Einstein able to address the meeting at the Albert Hall.
But Einstein and Szilard were exceptions. Most Jewish academics had not seen what was coming. Hans Bethe was born in German Alsace in 1906. He went on to win the 1967 Nobel Prize for Physics for explaining that sunshine is the result of the thermonuclear fusion of solar hydrogen.
Bethe's father was a university physiologist: his mother was Jewish. After his studies, he moved to a junior post in Tübingen, near Stuttgart. There he heard about the new Civil Service Law, and realised that it concerned him, as he had a Jewish mother.
However, the first direct impact came in a letter from a student, who had read in a newspaper that Bethe was being sacked. This was news to Bethe, who immediately wrote to his superior. A stiff reply confirmed the newspaper report.
B ethe wrote to his former teacher in Munich: "I ask your advice: you probably do not know that my mother is a Jewess, so according to the new laws I am not worthy of being an official of the Deutsches Reich… Where my 'birth mistake' (Geburtsfehler) is known and from where, I have no idea." Bethe did not know that Nazi enthusiasts had been carefully compiling lists of Jewish academics.
Although shocked by the suddenness of the Civil Service Law, Bethe was happy to discover that help was quickly available abroad. He first made his way to England, then across the Atlantic to the US.
Against this backdrop, just as the Nazis moved into power, nuclear physics was revolutionised by the discovery of a new nuclear component - the neutron. One of the main centres of this new neutron research was Berlin. Neutrons did strange thing but, in Berlin, a gifted scientist, Lise Meitner, always seemed to understand what was happening.
A century ago, women academics were treated with an arrogant contempt. Lise Meitner had to fight for unpaid positions. For years, she could not have lunch with her colleagues and, with her department in Berlin having no women's toilet, she had to frequent a nearby café.
She was also Jewish but, as an Austrian, had been exempt from the 1933 Civil Service Law. However, in 1938, Austria was annexed by Germany, and all Austrians technically became Germans. Meitner fled to Stockholm. Soon after she left, her former colleagues in Berlin discovered a mysterious effect when uranium was irradiated with neutrons. They wrote to Meitner in Stockholm: "Perhaps you can suggest some fantastic explanation".
Meitner had a young scientist nephew, Otto Frisch, who was also an Austrian working in Germany. Every Christmas, he went to visit his aunt in Berlin. But in 1938 he had to visit Stockholm instead. Together, they understood what Meitner's former colleagues in Berlin had discovered. Frisch then gave it a name. He called it "nuclear fission".
As the Jewish scientists who had fled Germany settled into new jobs, they realised how this nuclear fission held the key to a new source of energy. It could also be a weapon of unimaginable power, the atomic bomb. For them, it was not a great intellectual leap, so they were convinced that their former colleagues in Germany had come to the same conclusion.
Many of these exiles were now afraid. They still had families in Germany who had seen what the Nazis were prepared to do. When war looked imminent, the exiles had to get to the atomic bomb first. One of them wrote of "the fear of the Nazis beating us to it".
L eo Szilard, after emigrating for the fourth time in his life, was now in the US. There, he orchestrated an effort to launch an atomic bomb development programme. Szilard knew that President Roosevelt would not listen to him, but would probably listen to the most famous scientist in the world, Szilard's old friend, Albert Einstein. Szilard wrote a letter to Roosevelt and got Einstein to sign it.
After seeing the letter, Roosevelt ordered an effort "so that the Nazis don't blow us up". But nobody knew exactly what to do. The letter signed by Einstein talked about bombs "too heavy for transportation by air", so would have to be carried by boat. Such a threat did not appear urgent.
Nuclear fission pioneer Otto Frisch soon left Germany and in 1940 was working with another German Jewish exile, Rudolf Peierls, at the University of Birmingham. Uranium contained a small amount of the nuclear isotope 235. Frisch and Peierls saw that, if this could be separated, it could release an explosion equivalent to several thousand tons of dynamite.
Only a few kilograms of uranium 235 would be needed. Such a bomb could easily be carried by air, and the logistics of nuclear weapons suddenly changed. Szilard wrote another letter to Roosevelt and got Einstein to sign it. But, by this time, many others were pushing for the same thing.
On January 19 1942, Roosevelt ordered a crash programme for the development of the atomic bomb, the "Manhattan Project". Many emigrants from Europe were drafted in, and went on to play vital roles. They included Leo Szilard, Otto Frisch, Rudolf Peierls and Hans Bethe,
Across the Atlantic, the Germans had indeed seen the implications of nuclear fission - after all, they had discovered it. But its scientific message had been muffled. Many key scientists had gone, removed by the Civil Service Law. Germany had nobody left with the scientific insight of Szilard and the political clout of Einstein.
The Nazis also had another priority. In the opening years of the war, German armies overran the heartland of Jewry in Eastern Europe. Germany was preoccupied with its "Jewish problem" - what to do with the millions of Jews now under its jurisdiction. On January 20, one day after Roosevelt had given the go-ahead for the atomic bomb project, a top level meeting in the Berlin suburb of Wannsee outlined a "final solution of the Jewish Problem". Nazi Germany had its own crash programme.
Two huge projects, unknown to each other, emerged simultaneously on opposite sides of the Atlantic. The dreadful schemes forged ahead and each, in turn, became reality. On two counts, the unimaginable took on a dramatically imaginable shape.