A brief chronology of the atomic bomb

December 2, 1942:At the University of Chicago, scientists led by Italian-born Enrico Fermi create the first controlled nuclear chain reaction–a domino-like event in which the bombardment of a uranium atom’s nucleus by neutrons causes the nucleus to split, thereby scattering more neutrons that, in turn, split other nuclei. In the process, huge amounts of energy are released. This chain reaction is the foundation that will make atom bombs possible. Immediately, the US government establishes the Manhattan Project (named for the city where it was first headquartered), a secret research program overseen by Brigadier General Leslie R. Groves and charged with making a working atom bomb. Groves appoints physicist J. Robert Oppenheimer as the project’s scientific director. Headquarters is relocated to a custom-built laboratory at Los Alamos, New Mexico. The Manhattan Project will ultimately cost more than $2 billion.

Summer 1944: Special training begins in the United States for two B-29 Superfortress bomber crews and two backup crews, in preparation for the possible use of atom bombs against Japan. With the exception of the flyers’ leader, Colonel Paul W. Tibbets, none of the crewmen are aware of exactly what sort of bomb they are preparing to drop.

May 1945:The 20th Air Force’s 509th Bombardment Group (Composite)–a unit comprising the specially trained B-29 crewmen and their aircraft, organized by Tibbets–arrives on Tinian in the Mariana Islands–six and a half hours southeast of Japan by air. The crews will practice for their mission by dropping "pumpkins"–round, orange, 10,000-pound bombs full of TNT–on Japanese positions on Truk, in the Caroline Islands southeast of Tinian.

July 16, 1945: The first atom bomb in history is exploded atop a 100-foot steel tower at Alamogordo, New Mexico. The blast is a test to determine whether the bomb will work properly when deployed on an actual target. Scientists and military officials pronounce the test a success.

July 24, 1945: President Harry S. Truman officially decides to use nuclear warfare against Japan if the warring empire refuses to surrender.

July 25, 1945: Orders for Tibbets’s 509th Composite Group to "deliver its special bomb as soon as weather will permit" arrive at US Army Strategic Air Forces headquarters.

July 26, 1945: USS Indianapolis delivers a key component of the Little Boy atom bomb–a cup-shaped chunk of uranium-235–to Tinian. On the 30th, en route to the Philippines, the Indianapolis is torpedoed by a Japanese sub and sinks; 800 of the 1,200 men aboard manage to abandon ship. Three days will pass before 316 survivors are rescued—the other 484 will die in the water, many from shark attacks.

August 6, 1945: Tibbets and the crew of the B-29 Enola Gay drop the uranium-based atom bomb known as Little Boy on Hiroshima, Japan, a port and manufacturing city of about 350,000 on southwest Honshu, at 8:15 a.m. Between 70,000 and 80,000 people die instantly, and about as many are injured. Many tens of thousands more will die from radiation sickness and radiation-induced ailments in the days, months, and years after the blast.

August 9, 1945: Major Charles W. Sweeney and the crew of the B-29 Bock’s Car drop Fat Man, a plutonium-based atom bomb, on Nagasaki, Japan, a port city of a quarter-million inhabitants, on the west coast of Kyushu. About 45,000 are killed immediately. Within months, the death toll will reach about 70,000.

August 15, 1945: In a recorded speech broadcast throughout Japan, Emperor Hirohito announces that Japan is surrendering to the Allies. Among the reasons he gives for the surrender is that "the enemy, who has recently made use of an inhuman bomb, is incessantly subjecting innocent people to grievous wounds and massacre. The devastation is taking on incalculable proportions."

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