The Manhattan Project and the Invention of the Atomic Bomb

A nuclear weapon test by the American military at Bikini Atoll, Micronesia.
John Parrot/Stocktrek Images / Getty Images

During World War II, American physicists and engineers conducted a race against Nazi Germany to become the first to exploit the newly understood process of nuclear fission for military applications. Their secret endeavor, which lasted from 1942 to 1945, was known as the Manhattan Project.

The effort led to the invention of atomic bombs, including the 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.

The Project

The Manhattan Project was named for the Manhattan, New York, home of Columbia University, 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, occurred near Los Alamos, New Mexico.

For the project, the U.S. military teamed with the best minds of the scientific community. Military operations were headed by Brig. Gen. Leslie R. Groves, and physicist J. Robert Oppenheimer served as the scientific director, overseeing the project from concept to reality. The Manhattan Project cost the U.S. over $2 billion in just four years.

German Competition

In 1938, German scientists had 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.

Beginning in the late 1930s, a number of scientists, many escaping fascist regimes in Europe, immigrated to the U.S., bringing with them news of this discovery. In 1939, physicist Leo Szilard and other American and recently immigrated scientists tried to warn the U.S. government about this new danger but did not get a response. So Szilard contacted Albert Einstein, one of the best-known scientists of the day.

Einstein, a devoted pacifist, 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. Einstein was eventually swayed by concerns that Nazi Germany would develop the weapon first.

U.S. Government Gets Involved

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

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

The project was underway, but progress was slow until one fateful event brought the reality of war to American shores.

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 World War II.

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

Costly experiments began at the University of Chicago, the University of California Berkeley, and Columbia. Reactors, devices designed to initiate and control nuclear chain reactions, 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 to make the nuclear fuel.

Researchers worked simultaneously at all the sites to devise ways to produce the fuel. Physical chemist Harold Urey and his Columbia colleagues built an extraction system based on gaseous diffusion. At Berkeley, the inventor of the cyclotron, Ernest Lawrence, used his knowledge and skills to devise a process for magnetically separating the fuel: uranium-235 and plutonium-239 isotopes.

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

Site Consolidation

Another priority for the Manhattan Project 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. 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.

Trinity Test

When 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 atomic bomb development.

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

Everyone was anxious: Nothing of this magnitude had been tested before. No one knew what to expect. While some scientists feared a dud, others feared the end of the world.

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 extending 40,000 feet into the atmosphere. The tower from which the bomb was dropped disintegrated, and thousands of yards of surrounding desert sand was turned into a brilliant jade green radioactive glass.

The bomb was a success.


The bright light from the Trinity test stood out in the minds of everyone within hundreds of miles of the site that morning. Residents in faraway neighborhoods said the sun rose twice that day. A blind girl 120 miles from the site said she saw the flash.

The men who created the bomb were astonished. Physicist Isidor Rabi expressed worry that mankind had become a threat to 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." Physicist Ken Bainbridge, the test director, told Oppenheimer, "Now we're all sons of bitches."

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

2 A-Bombs End World War II

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

The war had lasted six years and involved most of the globe, resulting 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 bomb named “Little Boy” 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?"

Hiroshima A-Bomb Dome at sunset
traumlichtfabrik / Getty Images

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 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 flammable within two and a half miles was burned, and blazing infernos were seen up to three miles away.

On August 9, after Japan still refused to surrender, a second bomb was dropped, 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 Japanese who had survived the blast, and more lives were lost to radiation poisoning.

Survivors of the bombs passed radiation on to their descendants. The most prominent example was an alarmingly high rate of leukemia 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.


mla apa chicago
Your Citation
Schwartz, Shelly. "The Manhattan Project and the Invention of the Atomic Bomb." ThoughtCo, Apr. 5, 2023, Schwartz, Shelly. (2023, April 5). The Manhattan Project and the Invention of the Atomic Bomb. Retrieved from Schwartz, Shelly. "The Manhattan Project and the Invention of the Atomic Bomb." ThoughtCo. (accessed June 2, 2023).