In September 1930, Germany held its first national elections since the Great Crash of 1929, and the National Socialists won a stunning tally: 6,400,000 votes—10 times their total just two years before—and 107 seats. They were now the second largest party in the Reichstag. The word “Nazi” no longer evoked images of the madhouse, as one commentator wrote. Suddenly the party was almost respectable.

Even so, it still seemed to many as if Hitler’s support was tenuous. For Albert Einstein, Hitler’s sudden rush to prominence confirmed his historic distrust of the German body politic. But at this time, he did not see Hitler or National Socialism as a lasting danger. Asked in December of 1930 what to make of the new force in German politics, he answered that “I do not enjoy Herr Hitler’s acquaintance. He is living on the empty stomach of Germany. As soon as economic conditions improve, he will no longer be important.” Initially, he felt that no action at all would be needed to bring Hitler low. He reaffirmed for a Jewish organization that the “momentarily desperate economic situation” and the chronic “childish disease of the Republic” were to blame for the Nazi success. “Solidarity of the Jews, I believe, is always called for,” he wrote, “but any special reaction to the election results would be quite inappropriate.”

Einstein should have been right—the evidence for the fragility of Hitler’s support over the next two years makes for frustrating, bitter, what-if history. But even if he had persuasive reasons for believing that Hitler would not last, the election results reaffirmed the urgency of his core political stand. Even if he underestimated Hitler (as so many Germans did then), he still recognized the need to act to counter the more general pathology of which Hitler’s rise was a symptom.

The threat of German rearmament, along with a resurgence of militarism across the European continent spurred Einstein to act. Germany had been almost completely disarmed by the Versailles Treaty after World War I. Its army could total no more than 100,000 men; its forces were denied most heavy weapons; it could not build an air force; its warships had to meet strict tonnage and armament restrictions. Evasion of these terms had been the rule almost from the start.

This rearming barely a decade after a conflict that ought to have inoculated Germany against the contagion of battle-lust forever, was intolerable to Einstein. In response, he advocated mass rejection of compulsory military service by young men throughout Europe—a campaign that had become a major pillar of pacifist politics after the war. “Every thoughtful, well-meaning, and conscientious human being,” he wrote in January 1928 in a letter to London’s No More War movement, “should assume, in time of peace, the solemn and unconditional obligation not to participate in any war for any reason.”

He grew more insistent as time passed. In the spring of 1929, he wrote that “the people themselves must take the initiative to see to it that they will never again be led to slaughter. To expect protection from their governments is folly.” During the next several months 1930, driven by the rise of militant nationalism across Europe, Einstein’s level of urgency and passion grew. War had become an absolute anathema to him: “I would rather be torn limb from limb,” he wrote, “than take part in such an ugly business.”

By late 1932, Einstein abandoned the last of his hopes—or illusions—that a more or less democratic German society could survive economic collapse and the Nazi’s deliberate sabotage of civic life.

The Nazi setbacks in the November elections produced a brief moment of hope. Several quite acute political observers, including Einstein’s friend Kessler, thought that the Nazi losses marked the beginning of the end. But the moment evaporated, destroyed by Chancellor Fritz von Papen’s vacuous incompetence and Hitler’s relentless pursuit of power. Einstein had spoken at home and abroad against the collective surrender to unreason he saw around him. He had written, campaigned, served on committees, encouraged others, raised money when he could. But by late 1932, the end had clearly come.

From very early in his life, Einstein gave hints of a deep-seated streak of fatalism. It never prevented him from acting, from behaving as if what he sought to do could influence events. But the countervailing strain was always there, the perception that the apparently unique spark of any one human life must ultimately vanish into the vastness of the cosmos. The previous year, 1931, bound for California, he experienced a storm at sea. He wrote in his travel diary that “the sea has a look of indescribable grandeur, especially when the sun falls on it. One feels as if one is dissolved and merged into nature. Even more than usual, one feels the insignificance of the individual and it makes one happy.”

Insignificant—and hence autonomous, free to do what one had to do. In the end, Einstein simply left the stage. On December 12, Albert and Elsa Einstein set out from Berlin for the United States. A photograph taken at the entrance of the train station shows an ordinary travelers’ tableau. Elsa looks a little worried, harried; she could be thinking about the luggage, or perhaps, more seriously, about her daughter Ilse, who was ailing. Einstein’s face is unrevealing, almost grim. The overall impression is of impatience, a desire to be done with photography and catch their train. There is no way to read the image, except with hindsight, as the end of an era.

Before they reached the train station, Einstein and Elsa had to close up their house at Caputh. They may have paused at the door to Einstein’s study or on the porch, looking down the sweep of lawn to the lake, visible then through the leafless trees. There might have been a glance round the back of the house, a survey of windows shut and doors latched, and then in and out again, carrying their bags. One of them locked the door—probably Elsa, the master of all practical matters in the Einstein household. Finally, when nothing remained to be done, they walked away from the house. Einstein spoke. “Take a good look,” he told Elsa. “You will never see it again.”

* * *

In exile, Einstein rethought his core political beliefs and the moral reasoning that underpinned them. Being Einstein, he was faster to the conclusions that shift forced on him than almost all of his contemporaries.

On January 30, 1933, as Hitler took the oath as Chancellor of a republic about to become a Reich, Albert Einstein was safely out of reach in Pasadena. For the moment, there was little overt danger. Well treated by his American friends, he could be positively playful, even trying his hand at bicycling. The famous photograph of Einstein atop his two-wheeler was taken that February. He leans over, his front wheel a little askew. He seems a trifle unsteady but he grins hugely; life is pleasant in southern California.

Even after Hitler consolidated his hold, Einstein restrained himself for a while. Early in February, he even wrote to the Prussian Academy to discuss salary matters, fully as if he intended to resume work in Berlin later that year. But any illusions he may have had shattered almost immediately thereafter. On February 27, the Reichstag in Berlin burned to the ground. The crackdown on the left began immediately, with the SA and the SS competing to arrest and brutalize any perceived threat to the Reich.

By coincidence, the same day that Reichstag burned, Einstein wrote to his quondam mistress, Margarete Lenbach. He told her that “I dare not enter Germany because of Hitler.” …The day before he left Pasadena, bound eventually for Belgium, he launched his first public attack against Germany’s new regime. “As long as I have any choice in the matter, I shall live only in a country where civil liberty, tolerance, and equality of all citizens before the law prevail.” The completion of the syllogism was simple—”These conditions do not exist in Germany at the present time”—and would not, Einstein implied, as long as the current regime remained in power.

Hitler’s government reacted swiftly and bitterly to Einstein’s charges. The Völkicsher Beobachter published a series of attacks on him, and more mainstream papers followed suit. One headline read “Good News of Einstein—He Is Not Coming Back!” over an article condemning “this puffed-up bit of vanity [who] dared to sit in judgment on Germany without knowing what is going on here—matters that forever must remain incomprehensible to a man who was never a German in our eyes and who declares himself to be a Jew and nothing but a Jew.” A pamphlet that surfaced some months later reprinted Einstein’s photograph in a collection of enemies of Nazi Germany, over the caption, “Not Yet Hanged.”

Such harassment did not touch Einstein very deeply. The sharpest blows came not from the Nazis themselves but from those who had once formed his chief reason for being in Berlin, his fellow members of the Prussian Academy. While still at sea on the way to Belgium, Einstein drafted his letter of resignation from the Academy, and on arrival he gave it to the German legation, along with his renunciation of German citizenship.

Subsequent events revealed the depth to which the rot had spread. Hitler’s government ordered the Prussian Academy to begin the process of expelling Einstein from its midst. His resignation caught the government by surprise. Enraged that he had quit before he could be fired, the minister in charge demanded a proclamation from the Academy condemning its erstwhile hero. The draft statement declared that “we have no reason to regret Einstein’s resignation. The Academy is aghast at his foreign agitation.” Einstein’s old friend Max von Laue was horrified at the idea that the Academy might issue such a document, and he spoke against the proposal at an extraordinary meeting on April 6. Only one of the 14 members present supported him. Even Haber, the converted Jew and Einstein’s close friend, voted with the majority.

Haber’s action was bad. Max Planck disgraced himself. Einstein had written to Planck to refute privately the charge that he had spread rumors against Germany, telling him that he spoke now only to combat what was clearly a Nazi “war of extermination against my Jewish brethren.” Planck answered Einstein in a letter that identified both Jewishness and National Socialism as “ideologies that cannot co-exist. He deplored both and emphasized his loyalty to Germany, no matter who was in charge. “It is … greatly to be regretted,” he said at the Academy meeting, “that Mr. Einstein through his political behavior himself rendered his continued membership in the Academy impossible.” Einstein’s politics were to blame, not those of a German government that had chosen to destroy him.

Throughout the summer of 1933, Einstein sounded his warning about Hitler wherever he could. In September he visited Winston Churchill, then firmly in political exile—but while Churchill did not require much persuasion to view Hitler as a menace, he had no influence to bring to bear. Later that month, Einstein’s frustration became more obvious. “I cannot understand the passive response of the whole civilized world to this modern barbarism,” he told one interviewer. “Does the world not see that Hitler is aiming at war?”

That contained hints of the tectonic shift that had overtaken Einstein’s core political passion. By the time he spoke, he was no longer a pacifist. In September he had announced his change of heart in a letter to a Belgian war resister published in The New York Times. “Until quite recently we in Europe could assume that personal war resistance constituted an effective attack on militarism,” he began. But circumstances alter cases, and now, “in the heart of Europe lies a power, Germany, that is obviously pushing towards war with all available means.” For Einstein, even deeply held principles had to bend to the pressure of an overwhelming threat. “I should not, in the present circumstances, refuse military service,” he concluded. “Rather I should enter such service cheerfully in the belief that I would thereby be helping to save European civilization.”

The culmination of Einstein’s commitment to defeat Hitler by whatever means necessary came in 1939 and 1940, when he sent his two letters to President Roosevelt about the possibility of the United States building an atomic bomb. In late 1938, Otto Hahn and Fritz Strassman, two scientists still working in Berlin, were wrestling with some novel results from a series of experiments in which they bombarded uranium with a newly discovered subatomic particle, the neutron. Lise Meitner, Hahn’s former collaborator, and her nephew Otto Frisch, both exiles from Hitler’s Germany, met at Christmas in the Swedish village of Kungälv and together they identified the process the Berliners had observed: neutrons striking uranium atoms had sparked nuclear fission, the violent destruction of atomic nuclei in which both energy and more neutrons are released. The result was published several months before wartime secrecy would have rung the curtain down. Every competent physicist who heard the news realized that the fact that each fission event could release more neutrons, raised the possibility of a chain reaction, the new neutrons splitting more atoms in an escalating cascade. The next step was obvious even to the newspapers. As early as the spring of 1939, The Washington Post reported that nuclear fission could lead to weapons powerful enough to destroy everything over two square miles of ground.

In the first months after the fission experiments became public knowledge, however, Einstein had not paid much attention. During the summer of 1939, however, Szilard came to visit him at his summer house on Long Island, accompanied by his fellow physicists Eugene Wigner and Edward Teller. The three émigré Hungarians laid out the principle of the chain reaction, and then told Einstein of the interest the Germans were already showing in the use of uranium as a weapon. That was enough to persuade him to sign his first letter, in which he urged the president to consider the possibility of creating atomic weapons. Roosevelt replied in mid-October, saying that he had set up a committee to investigate Einstein’s suggestions. Nothing much happened—no surprise, given the initial committee budget of $6,000 for its first year of operation—so Szilard got Einstein to try again. In March, 1940, he sent his second letter to Roosevelt, urging him to give greater impetus to the effort because, Einstein wrote, “Since the outbreak of the war, interest in uranium has intensified in Germany. I have now learned that research there is carried out in great secrecy.”

Despite his attempt at presidential lobbying, and contrary to the often repeated fable that he was somehow the creator of the atom bomb, Einstein had next to nothing to do with the invention of nuclear weapons. The significance of his letters to Roosevelt was not the results they failed to achieve, but what they reveal about Einstein’s own political evolution. Until 1932, he had argued as fervently as he could that no civilized man should permit the state to order him to kill.

In the end, the use of America’s bombs deeply saddened him. On hearing of the attack on Hiroshima he is reported to have said “Oj Weg”—“Woe is me.” He later said that “had I known that the Germans would not succeed in producing an atomic bomb, I would not have lifted a finger.” After the war ended, Einstein became one of the founding forces in the scientists’ anti-nuclear movement. The last public act of his life was to add his name to a manifesto drafted by Bertrand Russell that called for global nuclear disarmament. But he never wavered in the basic argument he had made in the summer of 1933: Hitler was a deadly poison. He had to be neutralized. No greater goals could be contemplated until Hitler and Germany had been utterly defeated. Once he reached that conclusion, he followed it through to its ultimate destination: the bomb itself.

This article has been adapted from Thomas Levenson’s book, Einstein in Berlin.