In 1938, word came from Berlin that the nucleus of an atom could actually be split in two. This breakthrough was quickly confirmed in the United States and elsewhere. According to the theories of Albert Einstein, the fission of an atom should result in a release of energy. An atomic bomb was now no longer just science fiction -- it was a distinct possibility.

As the news of the fission breakthrough spread from Berlin in early 1939, many physicists immediately realized the potential danger posed by atomic energy. Especially concerned were émigré physicists who had fled their native countries because of the expansion of Nazi Germany and sought to obtain governmental support for further, secret nuclear research. Convincing busy government officials of the seriousness of this esoteric new scientific development was at first slow going. One month before the Second World War formally began with the September 1, 1939, invasion of Poland by Nazi Germany, Leo Szilard enlisted the help of Albert Einstein in personally calling President Franklin Roosevelt’s attention to the matter. Roosevelt responded by creating a government committee to coordinate and provide modest funding for early uranium research. Work also proceeded during this period on the design of an atomic pile that could demonstrate the potential of atomic energy and possibly provide a second path to the atomic bomb besides uranium.

Following the rapid successes of the German armies in Europe in 1940, many scientists felt that it was only a matter of time before the United States became involved in the war. They argued that reorganization and acceleration of atomic research was vital if a bomb was to be produced in time to affect the war. This belief was strengthened by the MAUD Report, the latest in a series of studies that argued that an atomic bomb was feasible. The surprise Japanese attack on Pearl Harbor of December 7, 1941, catapulted the United States into the war, and the following month Roosevelt secretly gave his tentative approval to proceed with the construction of an atomic bomb.

By early 1942, as the United States suffered a series of military defeats in the Pacific, top officials in Washington tentatively had decided to proceed with the construction of an atomic bomb. Two paths seemed possible. A uranium bomb could be achieved if sufficient uranium-235 could be produced by one or more of the three isotope separation methods under consideration: gaseous diffusion, centrifuge, and electromagnetic. A plutonium bomb might provide a quicker route, but it required demonstration that plutonium could be produced in a uranium pile and then be separated in usable quantities. To this end, most plutonium research was consolidated at the new Metallurgical Laboratory (Met Lab) at the University of Chicago.

A program review conducted in May 1942 determined that no front runner in the race for the bomb existed and recommended that the three isotope separation methods and the pile project be pushed as fast as possible to full production planning. Construction and security needs suggested placing the program in the Army Corps of Engineers. In August, the Corps set up the Manhattan Engineer District (MED) to manage the project. A month later, General Leslie Groves was appointed to head the effort. Groves moved quickly to move the project along, selecting a site in Oak Ridge, Tennessee, for the construction of production plants, dropping the centrifuge process from consideration, and choosing Robert Oppenheimer to head the bomb research and design laboratory to be built at Los Alamos, New Mexico. In December, President Roosevelt gave his final authorization to proceed with construction of the atomic bomb.

The uranium path to the atomic bomb ran through Oak Ridge. Only if the new plants built at Oak Ridge produced enough enriched uranium-235 would a uranium bomb be possible. General Groves placed two methods into production: 1) electromagnetic, based on the principle that charged particles of the lighter isotope would be deflected more when passing through a magnetic field; and 2) gaseous diffusion, based on the principle that molecules of the lighter isotope, uranium-235, would pass more readily through a porous barrier. Full-scale electromagnetic and gaseous diffusion production plants were built at Oak Ridge at sites designated as Y-12 and K-25.

The eventually-successful operations at both Y-12 and K-25, however, remained very much in doubt well into 1944. For this reason, the Army also implemented a liquid thermal diffusion method of uranium enrichment at the S-50 plant on the K-25 site as a supplement and a backup. In the end, it took the combined efforts of all three of these facilities to produce enough enriched uranium for the one and only uranium atomic bomb produced during the war.

Plutonium, produced in a uranium-fueled reactor (pile), was the second path taken toward achieving an atomic bomb. Design work on a full-scale plutonium production reactor began at the Met Lab in June 1942. General Groves selected a site at Hanford, Washington, on the Columbia River, to build the full-scale production reactors.

On December 2, 1942, on a racket court under the west grandstand at the University of Chicago’s Stagg Field, researchers headed by Enrico Fermi achieved the first self-sustaining chain reaction in a graphite and uranium pile known as CP-1. Three production reactors and corresponding chemical separation plants were built, with the first pile, the B Reactor, becoming operational in late September 1944. Los Alamos received its first plutonium from Hanford in early February 1945.

No matter how much enriched uranium and plutonium might be produced at Oak Ridge and Hanford, it would all come to nothing if workable weapon designs could not be developed in time. To this end, in late 1942 Groves established a bomb research and development laboratory at Los Alamos in the remote mountains of northern New Mexico. By 1944, it had become clear that, while a simple and reliable “gun-type” design could be used for a uranium bomb, the considerably more complicated implosion method would be required to produce a plutonium weapon. With the successful Allied landings in France on D-Day, June 6, 1944, the war in Europe appeared to be entering its final phase. Germany ceased to be the primary intended target. General Groves and his advisers turned their sights on Japan, and the rush was on to complete the atomic bomb in time to end the war in the Pacific.

Everything began to come together in the first months of 1945. Oak Ridge and Hanford produced enough enriched uranium and enough plutonium for at least one bomb using each. At Los Alamos bomb designs were finalized, and by the spring preparations had begun for the testing and use of the world’s first nuclear weapons. Meanwhile, word reached the Manhattan Project that Germany was not close to completing an atomic bomb. At the same time, espionage at Los Alamos was delivering critical weapon design information to the Soviet Union.

Security was a way of life for the Manhattan Project. The goal was to keep the entire atomic bomb program secret from Germany and Japan. In this, Manhattan Project security officials succeeded. They also sought, however, to keep word of the atomic bomb from reaching the Soviet Union. Although an ally of Britain and the United States in the war against Germany, the Soviet Union remained a repressive dictatorship and a potential future enemy. Here, security officials were less successful. Soviet spies penetrated the Manhattan Project at Los Alamos sending back to Russia critical information that helped speed the development of the Soviet bomb.

With an atomic bomb program of its own, Germany attempted to build a large spy network within the United States. Most German spies were quickly caught, however. German physicists heard rumors and suspected an atomic bomb project was underway in Britain, the United States, or both, but that was all. Japan also had a modest atomic research program. Rumors of the Manhattan Project reached Japan as well, but, as with Germany, no Japanese spies penetrated the Manhattan Project.

The Soviet Union proved more adept at espionage, primarily because it was able to play on the ideological sympathies of a significant number of Americans and British as well as foreign émigrés. In the US alone, hundreds of Americans provided secret information to the Soviet Union, and the quality of Soviet sources in Britain was even better. (In contrast, during the war neither the American nor the British secret services had a single agent in Moscow.) Soviet espionage directed at the Manhattan Project probably hastened by at least 12-18 months the Soviet acquisition of an atomic bomb. When the Soviet Union conducted its first nuclear test on August 29, 1949, the device they used was virtually identical in design to the one that had been tested at Trinity four years previously.

As the war entered its final phase, the Manhattan Project became an increasingly important and controversial element in American strategy. Debate over how to use the bomb began in earnest in early summer of 1945.

Some of President Truman’s advisers suggested that a demonstration of the atomic bomb might possibly convince the Japanese to surrender. This was rejected, however, out of fear that the bomb might be a dud, that the Japanese might put American prisoners of war in the area, or that they might manage to shoot down the plane. The shock value of the new weapon could also be lost. These reasons and others convinced the group that the bomb should be dropped without warning on a “dual target” -- a war plant surrounded by workers' homes. On June 6, Secretary of War Henry Stimson informed President Truman that his advisers recommended keeping the atomic bomb a secret until Japan had been bombed. The attack should take place as soon as possible and without warning.

The Trinity atomic test of July 16 confirmed that the stakes for this decision were very high. With a blast equivalent of approximately 21 kilotons of TNT, the test explosion was greater than had been predicted, and the dispersal of radioactive fallout following the test made safety real concern. News of the success at Trinity reached President Truman at the Potsdam Conference.

Following consultations with his advisers, Truman made the decision to use the bomb against Japan as soon as the first weapon was ready. Little Boy, the untested uranium bomb, was dropped first at Hiroshima, Japan, on August 6, 1945, while the plutonium weapon, Fat Man, followed three days later at Nagasaki on August 9. Use of the bomb helped bring an end to the war in the Pacific, with Japan surrendering on August 14. The most destructive war in human history was finally over. The Manhattan Project had fulfilled its mission.