“We knew Einstein was somewhere on Long Island, but we didn’t know precisely where,” Szilárd recalled. So he phoned Einstein’s Princeton, New Jersey, office and was told he was renting the house of a Dr. Moore in the village of Peconic. On Sunday, July 16, 1939, they embarked on their mission with Wigner at the wheel (Szilárd, like Einstein, did not drive). But when they arrived, they couldn’t find the house, and nobody seemed to know Dr. Moore. Then Szilárd saw a young boy standing by the curb. “Do you, by any chance, know where Professor Einstein lives?” he asked. Like most people in town, the boy did, and he led them up to a cottage near the end of Old Grove Road, where they found Einstein lost in thought.

Sitting at a wooden table on the porch of the sparsely furnished cottage, Szi­lárd explained how an explosive chain reaction could be produced in uranium layered with graphite by the neutrons released from nuclear fission: Those neutrons would split more nuclei, and so on. “I never thought of that!” Einstein interjected. He asked a few questions and quickly grasped the implications. Instead of writing the Belgian queen, Einstein suggested, they should contact a Belgian minister he knew.

Wigner, showing some sensible propriety, suggested that three refugees should not be writing a foreign government about secret security matters without consulting the U.S. State Department. Perhaps, they decided, the proper channel was a letter from Einstein (the only one of them famous enough to be heeded) to the Belgian ambassador, with a cover letter to the State Department. With that plan in mind, Einstein dictated a draft in German. Wigner translated it, gave it to his secretary to be typed, and then sent it to Szilárd.

A few days later, a friend arranged for Szilárd to talk to Alexander Sachs, an economist at Lehman Brothers and a friend of President Roosevelt’s. Showing a bit more savvy than the three theoretical physicists, Sachs insisted that the letter go right to the White House, and he offered to hand-deliver it.

It was the first time Szilárd had met Sachs, but he found the bold plan appealing. “It could not do any harm to try this way,” he wrote to Einstein. Einstein wrote back asking Szilárd to come back out to Peconic so they could revise the letter. By that point Wigner had gone to California for a visit. So Szilárd enlisted, as driver and scientific sidekick, another friend from the amazing group of Hungarian refugees who were theoretical physicists, Edward Teller.

Szilárd brought with him the original draft from two weeks earlier, but Einstein realized that they were now planning a letter that was far more momentous than one asking Belgian ministers to be careful about Congolese uranium exports. The world’s most famous scientist was about to tell the president of the United States that he should begin contemplating a weapon of almost unimaginable impact. “Einstein dictated a letter in German,” Szilárd recalled, “which Teller took down, and I used this German text as a guide in preparing two drafts of a letter to the president.”

According to Teller’s notes, Einstein’s dictated draft not only raised the question of the Congo’s uranium but also explained the possibility of chain reactions, suggested that a new type of bomb could result, and urged the president to set up formal contact with physicists working on this topic. Szilárd then prepared and sent back to Einstein a 45-line letter and a 25-line version—both dated August 2, 1939—“and left it up to Einstein to choose which he liked best.” Einstein signed them both in a small scrawl.

The scientists still had to figure out who could best get it into the hands of President Roosevelt. Einstein was unsure Sachs could do the job. When Szilárd sent back to Einstein the typed versions of the letter, he suggested that they use as their intermediary Charles Lindbergh, whose solo transatlantic flight 12 years earlier had made him a celebrity. All three refugee Jews were apparently unaware that the aviator had been spending time in Germany, had been decorated the year before by Hermann Göring with that nation’s medal of honor, and was becoming an isolationist and Roosevelt antagonist.

Einstein had briefly met Lindbergh a few years earlier in New York, so he wrote a note of introduction, which he included when he returned the signed letters to Szilárd. “I would like to ask you to do me a favor of receiving my friend Dr. Szilárd and think very carefully about what he will tell you,” Einstein wrote. “To one who is outside of science the matter he will bring up may seem fantastic. However, you will certainly become convinced that a possibility is presented here which has to be very carefully watched in the public interest.”

Lindbergh did not respond, so Szilárd wrote him a reminder letter on September 13. Two days later, he realized how clueless he and his colleagues had been when Lindbergh gave a nationwide radio address. It was a clarion call for isolationism. “The destiny of this country does not call for our involvement in European wars,” Lindbergh began. Interwoven were hints of his pro-German sympathies and even some anti-Semitic implications about Jewish ownership of the media. “We must ask who owns and influences the newspaper, the news picture, and the radio station,” Lindbergh said. “If our people know the truth, our country is not likely to enter the war.”

Szilárd’s next letter to Einstein stated the obvious. “Lindbergh is not our man,” he wrote.

The physicists’ other hope was Sachs, who had been given the formal letter to Roosevelt that Einstein signed. But Sachs was not able to find the opportunity to deliver it for almost two months.

By then, events had turned what had been an important letter into an urgent one. At the end of August 1939, the Nazis and Soviets stunned the world by signing a war-alliance pact and proceeded to carve up Poland. That prompted Britain and France to declare war.

Szilárd went to see Sachs in late September and was horrified to discover that he still had not been able to schedule an appointment with Roosevelt. “There is a distinct possibility Sachs will be of no use to us,” Szilárd wrote to Einstein. “Wigner and I have decided to accord him ten days’ grace.” Sachs barely made the deadline. On the afternoon of Wednesday, October 11, he was ushered into the Oval Office carrying Einstein’s letter, Szilárd’s memo, and an 800-word summary he had written on his own.

The president greeted him jovially: “Alex, what are you up to?”

Sachs worried that if he simply left Einstein’s letter and the other papers with Roosevelt, they might be glanced at and then pushed aside. The only reliable way to deliver them, he decided, was to read them aloud. Standing in front of the president’s desk, he read his summation of Einstein’s letter and parts of Szilárd’s memo.

“Alex, what you are after is to see that the Nazis don’t blow us up,” the president said.

“Precisely,” Sachs replied.

“This requires action,” Roosevelt declared to his assistant.

The following week, Einstein received a polite and formal thank-you letter from the president. “I have convened a board,” Roosevelt wrote, “to thoroughly investigate the possibilities of your suggestion regarding the element of uranium.” Still, the effort’s slow pace and meager funding prompted Szilárd and Einstein to compose a second letter urging the president to consider whether the American work was proceeding quickly enough.

Despite helping to spur Roosevelt into action, Einstein never worked directly on the bomb project. J. Edgar Hoover, the director of the FBI even back then, wrote a letter to General Sherman Miles, who initially organized the efforts, that described Einstein’s pacifist activities and suggested that he was a security risk. In the end, Einstein played only a small role in the Manhattan Project. He was asked by Vannevar Bush, one of the project’s scientific overseers, to help on a specific problem involving the separation of isotopes that shared chemical traits. Einstein was happy to comply. Drawing on his old expertise in osmosis and diffusion, he worked for two days on a process of gaseous diffusion in which uranium was converted into a gas and forced through filters.

The scientists who received Einstein’s report were impressed, and they discussed it with Bush. In order for Einstein to be more useful, they said, he should be given more information about how the isotope separation fit in with other parts of the bomb-making challenge. Bush refused. He knew that Einstein didn’t have and couldn’t get the necessary security clearance. “I wish very much that I could place the whole thing before him and take him fully into confidence,” Bush wrote, “but this is utterly impossible in view of the attitude of people here in Washington who have studied his whole history.”

Thus the scientist who had explained the need for a bomb-making project was considered too risky to be told about it.