The Manhattan Project and the Invention of the Atomic Bomb

The Manhattan Project and the Invention of the Atomic Bomb

During World War II, American physicists and engineers began a race against Nazi Germany to develop the first atomic bomb. Their secret endeavor, which lasted from 1942 to 1945, was known as the Manhattan Project.

The project led to the invention of nuclear weapons, including two that were dropped on the Japanese cities of Hiroshima and Nagasaki, killing or injuring over 200,000 people. These attacks forced Japan to surrender and brought an end to World War II, but they also marked a crucial turning point in the early Atomic Age, raising enduring questions about the implications of nuclear warfare.

What Was the Manhattan Project?

The Manhattan Project was named for Columbia University in Manhattan, New York, one of the initial sites of atomic study in the United States. While the research took place at several secret sites across the U.S., much of it, including the first atomic tests, took place near Los Alamos, New Mexico.

During the project, the U.S. military teamed up with the best minds of the scientific community. Military operations were headed by Brigadier General Leslie R. Groves, and J. Robert Oppenheimer acted as the scientific director, overseeing the project from concept to reality.

In total, the Manhattan Project cost the U.S. over two billion dollars over just four years.

A Race Against the Germans

In 1938, German scientists discovered fission, which occurs when the nucleus of an atom breaks into two equal parts. This reaction releases neutrons that break up more atoms, causing a chain reaction. Since significant energy is released in only millionths of a second, it was thought that fission could cause an explosive chain reaction of considerable force inside a uranium bomb.

Due to the war, a number of scientists emigrated from Europe and brought with them news of this discovery. In 1939, Leo Szilard and other American and recently emigrated scientists tried to warn the U.S. government about this new danger-but were not able to get a response. Szilard contacted and met with Albert Einstein, one of the best-known scientists of the day.

Einstein was a devoted pacifist and was at first reluctant to contact the government. He knew that he would be asking them to work toward creating a weapon that could potentially kill millions of people. However, Einstein was eventually swayed by concerns that Nazi Germany would develop the weapon first.

The Advisory Committee on Uranium

On August 2, 1939, Einstein wrote a now-famous letter to President Franklin D. Roosevelt. It outlined both the potential uses of an atomic bomb and ways to help support American scientists in their research. In response, President Roosevelt created the Advisory Committee on Uranium in October 1939.

Based on the recommendations of the committee, the U.S. government outlaid $6,000 to buy graphite and uranium oxide for research. Scientists believed that graphite might be able to slow down a chain reaction, thus keeping the bomb's energy somewhat in check.

Despite immediate action being taken, progress was slow until one fateful event brought the reality of war to American shores.

The Development of the Bomb

On December 7, 1941, the Japanese military bombed Pearl Harbor, Hawaii, the headquarters of the United States Pacific Fleet. In response, the U.S. declared war on Japan the next day and officially entered WWII.

With the country at war and the realization that the United States was now three years behind Nazi Germany, President Roosevelt was ready to seriously support U.S. efforts to create an atomic bomb.

Costly experiments began at the University of Chicago, U.C. Berkeley, and Columbia University in New York. Reactors were built in Hanford, Washington, and Oak Ridge, Tennessee. Oak Ridge, known as "The Secret City," was also the site of a massive uranium enrichment laboratory and plant.

Researchers worked simultaneously at all of the sites. Harold Urey and his Columbia University colleagues built an extraction system based on gaseous diffusion. At the University of California in Berkley, the inventor of the Cyclotron, Ernest Lawrence, used his knowledge and skills to devise a process for magnetically separating the uranium-235 (U-235) and plutonium-239 (Pu-239) isotopes.

Research kicked into high gear in 1942. On December 2, at the University of Chicago, Enrico Fermi created the very first successful chain reaction, in which atoms were split in a controlled environment. This accomplishment gave renewed vigor to the hopes that an atomic bomb was possible.

The Manhattan Project had another priority that soon became clear. It was becoming too dangerous and difficult to develop nuclear weapons at these scattered universities and towns. Scientists needed an isolated laboratory away from the populace.

In 1942, Oppenheimer suggested the remote area of Los Alamos, New Mexico. General Groves approved the site and construction began at the end of that year. Oppenheimer became the director of the Los Alamos Laboratory, which would be known as “Project Y."

Scientists continued to work diligently, but it took until 1945 to produce the first nuclear bomb.

The Trinity Test

When President Roosevelt died on April 12, 1945, Vice President Harry S. Truman became the 33rd President of the United States. Until then, Truman had not been told of the Manhattan Project, but he was quickly briefed on the secrets of the atomic bomb development.

That summer, a test bomb codenamed "The Gadget" was taken to the New Mexico desert, to a location known as Jornada del Muerto, Spanish for "Journey of the Dead Man." The test was given the codename “Trinity,” a name chosen by Oppenheimer in reference to a poem by John Donne.

Having never tested anything of this magnitude before, everyone was anxious. While some scientists feared a dud, others feared the end of the world. No one knew what to expect.

At 5:30 a.m. on July 16, 1945, scientists, army personnel, and technicians donned special goggles to watch the beginning of the Atomic Age. The bomb was dropped.

There was a forceful flash, a wave of heat, a stupendous shock wave, and a mushroom cloud that extended 40,000 feet into the atmosphere. The tower from which the bomb was dropped was completely disintegrated and thousands of yards of surrounding desert sand was turned into a radioactive glass of a brilliant jade green color.

The bomb was a success.

Reactions to the First Atomic Test

The bright light from the Trinity test would stand out in the minds of everyone who was within even hundreds of miles of the site that morning. Residents in neighborhoods far away would say the sun rose twice that day. A blind girl 120 miles from the site said she saw the flash as well.

The men who created the bomb were astonished, too. Physicist Isidor Rabi expressed worry that mankind had become a threat and upset the equilibrium of nature. The test brought to Oppenheimer's mind a line from the Bhagavad Gita: "Now I am become death, the destroyer of worlds." Test director Ken Bainbridge told Oppenheimer, "Now we're all sons of bitches."

The unease among many of the witnesses that day led some to sign petitions. They argued that this terrible thing they had created could not be let loose in the world. Their protests were ignored.

The Atomic Bombs That Ended WWII

Germany surrendered on May 8, 1945, two months before the successful Trinity test. However, Japan refused to surrender, despite threats from President Truman that terror would fall from the sky.

The war had lasted six years and involved most of the globe. It had resulted in the deaths of 61 million people and the displacement of countless others. The last thing the U.S. wanted was a ground war with Japan-so the decision was made to drop an atomic bomb.

On August 6, 1945, a uranium bomb named “Little Boy” (named for its relatively small size) was dropped on Hiroshima, Japan by the Enola Gay. Robert Lewis, co-pilot of the B-29 bomber, wrote in his journal moments later, "My God, what have we done?"

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The target of Little Boy was the Aioi Bridge, which spanned the Ota River. At 8:15 that morning the bomb was dropped, and by 8:16 over 66,000 people near ground zero were already dead. Some 69,000 more were injured, most burned or suffering from radiation sickness, from which many would later die.

This single atomic bomb produced absolute devastation. It left a "total vaporization" zone of one-half mile in diameter. The "total destruction" area extended to one mile, while the impact of a "severe blast" was felt for two miles. Anything that was flammable within two and a half miles was burned, and blazing infernos were seen up to three miles away.

On August 9, 1945, after Japan had still refused to surrender, a second bomb was dropped. It was a plutonium bomb named “Fat Man” after its round shape. The bomb's target was the city of Nagasaki, Japan. Over 39,000 people were killed and 25,000 injured.

Japan surrendered on August 14, 1945, bringing an end to World War II.


The deadly impact of the atomic bomb was immediate, but the effects would last for decades. The fallout caused radioactive particles to rain on the injured Japanese people who had survived the blast, and more lives were lost to the effects of radiation poisoning.

Survivors of these bombs would also pass radiation on to their descendants. The most prominent example is an alarmingly high rate of leukemia cases among their children.

The bombings at Hiroshima and Nagasaki revealed the true destructive power of these weapons. Though countries throughout the world have continued to develop nuclear weapons, there have also been movements to promote nuclear disarmament, and anti-nuclear treaties have been signed by major world powers.