The Manhattan project: how a secret U.S. plan to build a powerful new weapon 70 years ago forever changed the world.

• Position: TIME PAST: 1942

On Sept. 22, 1942, in the middle of World War II, The New York Times reported the promotion of 104 members of the armed forces. Few readers likely noticed the name of Colonel Leslie R. Groves of the Corps of Engineers, who was bumped up to brigadier general. His new assignment wasn't announced, but the next day, he took command of a top-secret mission: to transform the principles of theoretical physics into a devastating new weapon that could be delivered by plane against Nazi Germany and Japan.

What would later become known as the Manhattan Project would employ 130,000 people at secret sites in New York; Oak Ridge, Tennessee; Hanford, Washington; and Los Alamos, New Mexico--where the bombs the U.S. dropped on Hiroshima and Nagasaki in 1945 to end the war with Japan were made.

The Atomic Age, ushered in by the Manhattan Project, literally changed the world. The race for nuclear supremacy became a driving force in the Cold War--the decades-long confrontation between the U.S. and the Soviet Union after World War II--and raised the specter of nuclear annihilation for the entire world.

"Nuclear weapons put in pretty substantial doubt the long-term survival of civilization," says James G. Hershberg, a history professor at George Washington University.

The Manhattan Project was rooted in a letter signed by the physicist Albert Einstein and sent to President Franklin D. Roosevelt in August 1939, a month before Germany's invasion of Poland set off World War II. Drawing on his famous equation E = [mc.sup.2] (see timeline, p. 20), Einstein and other scientists told FDR about research in Europe and at American universities that demonstrated how a chain reaction involving uranium could unleash tremendous power that could be harnessed in "extremely powerful bombs." (For a copy of the letter, go to www.upfrontmagazine.com.)

"The letter expresses the threat of the Germans succeeding," says Cynthia C. Kelly, president of the Atomic Heritage Foundation. "Everyone knew that Hitler, if he had an atomic bomb, would not hesitate to use it on London or Washington."

Pearl Harbor

As early as 1919, a scientist in Britain had found a way to transform atoms of nitrogen into oxygen. Further research over the next two decades revealed that when an atom's nucleus is split (fission) or combined with other atoms (fusion), vast amounts of energy are released. That touches off a chain reaction as the process repeats itself. Scientists thought uranium and the newly discovered element of plutonium were most likely to produce that reaction--in theory, anyway.

Roosevelt heeded Einstein's warning and appointed a panel of scientists to pursue atomic research and development. A year and a half later, on Dec. 7, 1941, the Japanese bombed Pearl Harbor, the U.S. naval base in Hawaii. That brought the U.S. into the war in both Europe and Asia and added urgency to the race for nuclear weapons.

The name Manhattan Project came from the mission's first headquarters in New York City. (Manhattan is one of the five boroughs of New York.) In addition to a strong military presence, New York at the time was home to many scientists who had fled Hitler, and its vast harbor could receive uranium ore and other precious cargo.

The project recruited leading physicists, including refugees from fascist Europe like Niels Bohr of Denmark and Enrico Fermi of Italy. The physicist J. Robert Oppenheimer was in charge of designing and building the bomb in collaboration with British scientists. Oppenheimer told his colleagues that this megaweapon might prevent future world wars.

Supplies of uranium were secured, and secret factories were built to process it...