With the U. S. formal engagement in World War II and the elevation of nuclear research to highest priority, secrecy of all aspects of the project became absolute. First, no physicist could publish papers having to do with nuclear fission, so as not to reveal to external governments that bomb research was proceeding. The sudden stoppage of any nuclear publications only indicated to the Soviet Union, always sensitive to West­ern secrets, that something was going on in the United States, and they immediately started an infiltration effort.

immense laboratory complexes were built in remote, uninhabited locations and given names “Site X" and “Site Y.” Workers never knew what they were building, industrial workers at production facilities were never told what they were producing and were forbidden from talking to the workers standing next to them, much less anyone outside the building or from another building. There were words that could not be said. No one could say uranium-235. The material was referred to as “oral — loy.” One could never refer to the uranium isotope separation plant as anything but Y-12, K-25, or S-50. Plants and laboratories were ringed with high fences, barbed wire, and armed watchtowers. Photography was highly restricted, and every piece of paper had to go in a combination lock safe or vault if it was not being written on or read from. There was a counter-story written to explain every odd sound, flash of light, or truck leaving a hidden facility. Foreign scientists working on the project, of which there were many, could find themselves being tailed by security operatives if they left the laboratory grounds.

Scientists were given working names, which were printed on their security badges. Enrico Fermi was Mr. Farmer and Niels Bohr was Mr. Baker. Once Bohr wound up at a checkpoint without his badge, but Fermi was able to vouch for him, saying, “I assure you that this is Mr. Baker, sure as my name is Mr. Farmer.” Scientists disappeared from their university positions and were not heard from again until after the war. The general public in the United States had no idea that atomic weapons were being developed in their country. Knowledge was dispensed only on a strict need-to-know basis. The vice president of the United States, Harry S. Truman (1884-1972), was not advised that his country was developing an atomic bomb until he was sworn in as the commander in chief on Franklin Roosevelt’s death on April 12, 1945. Neither the Germans nor the Japanese knew anything about the U. S. bomb project and did not know to be concerned.

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The British were also very aware of the need for atomic security, and they care­fully monitored all correspondence and radio traffic into and out of the home islands. When Copenhagen was occupied by German forces, Niels Bohr sent a message to a German physicist working in England, mentioning a “Miss Maud Rey at Kent.” Think­ing that it was surely a coded message, the British code masters found that the let­ters could be rearranged to form “radium taken.” Miss Maud Rey turned out to be the former governess of Bohr’s children, and she was indeed living in Kent. There was no coded message. The MAUD Committee was named in her honor.

already be engaging in nuclear work. On August 15, Szilard transmitted the document to Sachs with the authoritative Einstein signature.

Sachs knew that the concept had to be sprung on the president at just the right time, when he was in the correct mood, and unfortunately events in Europe were monopolizing his attention. Weeks passed. Finally, as Szi­lard’s patience was at the boil, Sachs saw the opportunity to approach the president and present the letter on October 11 at the White House in the Oval Office. Neither man actually read the letter, but Sachs gave Roosevelt an 800-word synopsis of what he had learned from Szilard.

Roosevelt absorbed it all and summed it up, saying, “Alex, what you are after is to see that the Nazis don’t blow us up.”

“Precisely,” said Sachs.

Roosevelt called in his aide, saying, “This requires action.”

So began the largest scientific research and development program in history. It would take another letter from Einstein five months later on April 25, 1940, to speed things along, but the ball was now rolling, and there was no stopping it.