President Truman is briefed on Manhattan Project

President Harry S. Truman learns the full details of the Manhattan Project, in which scientists are attempting to create the first atomic bomb, on April 24, 1945. The information thrust upon Truman a momentous decision: whether or not to use the world’s first weapon of mass destruction.

America’s secret development of the atomic bomb began in 1939 with then-President Franklin Roosevelt’s support. The project was so secret that FDR did not even inform his fourth-term vice president, Truman, that it existed. (In fact, when Truman’s 1943 senatorial investigations into war-production expenditures led him to ask questions about a suspicious plant in Minneapolis, which was secretly connected with the Manhattan Project, Truman received a stern phone call from FDR’s secretary of war, Harry Stimson, warning him not to inquire further.)

When President Roosevelt died on April 12, 1945, Truman was immediately sworn in and, soon after, was informed by Stimson of a new and terrible weapon being developed by physicists in New Mexico. In his diary that night, Truman noted that he had been informed that the U.S. was perfecting an explosive great enough to destroy the whole world.

READ MORE: “Father of the Atomic Bomb” Was Blacklisted for Opposing H-Bomb

On April 24, Stimson and the army general in charge of the project, Leslie Groves, brought Truman a file full of reports and details on the Manhattan Project. They told Truman that although the U.S. was the only country with the resources to develop the bomb–eliminating fears that Germany was close to developing the weapon–the Russians could possibly have atomic weapons within four years. They discussed if, and with which allies, they should share the information and how the new weapon would affect U.S. foreign-policy decisions. Truman authorized the continuation of the project and agreed to form an interim committee that would advise the president on using the weapon.

Although the war in Europe ended in May 1945, Stimson advised Truman that the bomb might be useful in intimidating Soviet leader Joseph Stalin into curtailing post-war communist expansion into Eastern Europe. On July 16, the team of scientists at the Alamogordo, New Mexico, research station successfully exploded the first atomic bomb. Truman gave Stimson the handwritten order to release when ready but not sooner than August 2 on July 31, 1945.

The first bomb was exploded over Hiroshima on August 6, 1945, and a second was dropped on Nagasaki on August 9. The Japanese quickly surrendered. Although other nations have developed atomic weapons and nuclear technology since 1945, Truman remains the only world leader to have ever used an atomic bomb against an enemy.

READ MORE: The Hiroshima Bombing Didn't Just End WWII—It Kick-Started the Cold War

On April 12, only weeks before Germany's unconditional surrender on May 7, President Roosevelt died suddenly in Warm Springs, Georgia, bringing Vice President Harry S. Truman, a veteran of the United States Senate, to the presidency. Truman was not privy to many of the secret war efforts Roosevelt had undertaken and had to be briefed extensively in his first weeks in office. One of these briefings, provided by Secretary of War Stimson on April 25, concerned S-1 (the Manhattan Project). Stimson, with Groves present during part of the meeting, traced the history of the Manhattan Project, summarized its status, and detailed the timetable for testing and combat delivery. Truman asked numerous questions during the forty-five minute meeting and made it clear that he understood the relevance of the atomic bomb to upcoming diplomatic and military initiatives.

By the time Truman took office, Japan was near defeat. American aircraft were attacking Japanese cities at will. A single fire bomb raid in March killed nearly 100,000 people and injured over a million in Tokyo. A second air attack on Tokyo in May killed 83,000. Meanwhile, the United States Navy had cut the islands' supply lines. But because of the generally accepted view that the Japanese would fight to the bitter end, a costly invasion of the home islands seemed likely, though some American policy makers held that successful combat delivery of one or more atomic bombs might convince the Japanese that further resistance was futile.

Former ANS president Wilkins’s role in Manhattan Project highlighted

ANS past president (1974–1975) J. Ernest Wilkins Jr. was featured in a recent article highlighting the unsung contributions that Black scientists made to the Manhattan Project.

At least 12 Black chemists and physicists participated in primary research at the Chicago Metallurgical Laboratory for the Manhattan Project, a small fraction of the more than 400 scientists, technicians, and laboratory staff members tasked with designing a method of plutonium production that could fuel a nuclear reaction. Wilkins joined the team at the Met Lab in 1944 when he was only 21 years old (he had entered the University of Chicago at the age of 13 and earned his bachelor’s, master’s, and Ph.D. degrees in six years).

Left behind: At the Met Lab, Wilkins researched neutron energy, reactor physics, and engineering with two prominent European-born scientists, Enrico Fermi and Leo Szilard. Together they did groundbreaking work in the movement of subatomic particles. But when his team was transferred in 1944 to Oak Ridge, Tenn., a Manhattan Project site where the X-10 Graphite Reactor was being built, Wilkins was left behind because he was Black, according to the article.

More details: At the Met Lab, Wilkins joined the Atomic Scientists of Chicago, which was founded in 1945 to address the moral and social responsibilities of scientists regarding the use of the atomic bomb. Wilkins was one of 70 Manhattan Project scientists who signed a petition urging President Truman to not use the atomic bomb on Japan without first demonstrating its power and giving Japan the option to surrender. But Truman never saw the petition, which didn’t become widely known about until it was declassified in 1961.

After World War II, Wilkins worked for a decade as a mathematician at United Nuclear Corporation. He later went on to distinguished professorships at two historically Black universities, Howard University and Clark Atlanta University, before his retirement in 2003.

When he died in 2011 at the age of 87, Wilkins had authored more than 100 scholarly papers. According to Shane Landrum, a historian of Black atomic scientists, the work of Wilkins and other Black Manhattan Project scientists, along with their white and immigrant colleagues, changed the “course of the war and the role of science in American politics.”

Truman Statement on Hiroshima

President Harry Truman issued this statement after the first atomic bomb was dropped on Hiroshima. His statement unveiled the top secret Manhattan Project and portrays it as an immense success in the history of science and warfare. President Truman envisions the production and use of atomic energy for power within the United States and as a force for maintaining world peace.



Sixteen hours ago an American airplane dropped one bomb on Hiroshima, an important Japanese Army base. That bomb had more power than 20,000 tons of T.N.T. It had more than two thousand times the blast power of the British “Grand Slam” which is the largest bomb ever yet used in the history of warfare.

The Japanese began the war from the air at Pearl Harbor. They have been repaid many fold. And the end is not yet. With this bomb we have now added a new and revolutionary increase in destruction to supplement the growing power of our armed forces. In their present form these bombs are now in production and even more powerful forms are in development.

It is an atomic bomb. It is a harnessing of the basic power of the universe. The force from which the sun draws its power has been loosed against those who brought war to the Far East.

Before 1939, it was the accepted belief of scientists that it was theoretically possible to release atomic energy. But no one knew any practical method of doing it. By 1942, however, we knew that the Germans were working feverishly to find a way to add atomic energy to the other engines of war with which they hoped to enslave the world. But they failed. We may be grateful to Providence that the Germans got the V-1’s and the V-2’s late and in limited quantities and even more grateful that they did not get the atomic bomb at all.

The battle of the laboratories held fateful risks for us as well as the battles of the air, land, and sea, and we have now won the battle of the laboratories as we have won the other battles.

Beginning in 1940, before Pearl Harbor, scientific knowledge useful in war was pooled between the United States and Great Britain, and many priceless helps to our victories have come from that arrangement. Under that general policy the research on the atomic bomb was begun. With American and British scientists working together we entered the race of discovery against the Germans.

The United States had available the large number of scientists of distinction in the many needed areas of knowledge. It had the tremendous industrial and financial resources necessary for the project and they could be devoted to it without undue impairment of other vital war work. In the United States the laboratory work and the production plants, on which a substantial start had already been made, would be out of reach of enemy bombing, while at that time Britain was exposed to constant air attack and was still threatened with the possibility of invasion. For these reasons Prime Minister Churchill and President Roosevelt agreed that it was wise to carry on the project here. We now have two great plants and many lesser works devoted to the production of atomic power. Employment during peak construction numbered 125,000 and over 65,000 individuals are even now engaged in operating the plants. Many have worked there for two and a half years. Few know what they have been producing. They see great quantities of material going in and they see nothing coming out of those plants, for the physical size of the explosive charge is exceedingly small. We have spent two billion dollars on the greatest scientific gamble in history—and won.

But the greatest marvel is not the size of the enterprise, its secrecy, nor its cost, but the achievement of scientific brains in putting together infinitely complex pieces of knowledge held by many men in different fields of science into a workable plan. And hardly less marvelous has been the capacity of industry to design, and of labor to operate, the machines and methods to do things never done before so that the brain child of many minds came forth in physical shape and performed as it was supposed to do. Both science and industry worked under the direction of the United States Army, which achieved a unique success in managing so diverse a problem in the advancement of knowledge in an amazingly short time. It is doubtful if such another combination could be got together in the world. What has been done is the greatest achievement of organized science in history. It was done under high pressure and without failure.

We are now prepared to obliterate more rapidly and completely every productive enterprise the Japanese have above ground in any city. We shall destroy their docks, their factories, and their communications. Let there be no mistake we shall completely destroy Japan’s power to make war.

It was to spare the Japanese people from utter destruction that the ultimatum of July 26 was issued at Potsdam. Their leaders promptly rejected that ultimatum. If they do not now accept our terms they may expect a rain of ruin from the air, the like of which has never been seen on this earth. Behind this air attack will follow sea and land forces in such numbers and power as they have not yet seen and with the fighting skill of which they are already well aware.

The Secretary of War, who has kept in personal touch with all phases of the project, will immediately make public a statement giving further details.

His statement will give facts concerning the sites at Oak Ridge near Knoxville, Tennessee, and at Richland near Pasco, Washington, and an installation near Santa Fe, New Mexico. Although the workers at the sites have been making materials to be used in producing the greatest destructive forces in history they have not themselves been in danger beyond that of many other occupations, for the utmost care has been taken of their safety.

The fact that we can release atomic energy ushers in a new era in man's understanding of nature's forces. Atomic energy may in the future supplement the power that now comes from coal, oil, and falling water, but at present it cannot be produced on a basis to compete with them commercially. Before that comes there must be a long period of intensive research.

It has never been the habit of the scientists of this country or the policy of this Government to withhold from the world scientific knowledge. Normally, therefore, everything about the work with atomic energy would be made public.

But under present circumstances it is not intended to divulge the technical processes of production or all the military applications, pending further examination of possible methods of protecting us and the rest of the world from the danger of sudden destruction.

I shall recommend that the Congress of the United States consider promptly the establishment of an appropriate commission to control the production and use of atomic power within the United States. I shall give further consideration and make further recommendations to the Congress as to how atomic power can become a powerful and forceful influence towards the maintenance of world peace.

Tag: Manhattan Project

The 10,000lb, 10𔄂″ weapon was released at 28,900′. Seconds later, a perfect circle of 64 detonators exploded inside the heart of the bomb, compacting the plutonium core into a supercritical mass and exploding with the force of 20,000 tons of high explosive.

Now I am become death, the destroyer of worlds“.

Trinity Test Fireball

The line comes from the Bhagavad Gita, the Hindu epic which Mohandas Gandhi described as his “spiritual dictionary”. On July 16, 1945, these words were spoken by J. Robert Oppenheimer, director of the Manhattan Project, as he witnessed “Trinity”, the world’s first nuclear detonation.

The project had begun with a letter from prominent physicists Leo Szilard and Albert Einstein to President Franklin Roosevelt, warning that Nazi Germany may have been working to develop a secret “Super Weapon”. The project ended with the explosion of the “Gadget” in the Jornada del Muerto desert, equaling the explosive force of 20 kilotons of TNT.

The Manhattan Project, the program to develop the Atomic Bomb, was so secret that even Vice President Harry Truman was unaware of its existence.

President Roosevelt passed away on April 14, and Harry Truman was immediately sworn in as President. He was fully briefed on the Manhattan project 10 days later, writing in his diary that night that the US was perfecting an explosive “great enough to destroy the whole world”.

Nazi Germany surrendered on May 7, but the war in the Pacific theater, ground on. By August, Truman faced the most difficult decision ever faced by an American President. Whether to drop an atomic bomb on Imperial Japan.

The morality of President Truman’s decision has been argued ever since. In the end, it was decided that to drop the bomb would end the war faster, with less loss of life on both sides, compared with the invasion of the Japanese home islands.

So it was that the second nuclear detonation in history took place on August 6 over the city of Hiroshima, Japan. “Little Boy”, as the bomb was called, was delivered by the B29 Superfortress “Enola Gay”, named after the mother of United States Army Air Forces pilot Lieutenant Colonel Paul Tibbets. 70,000 Japanese citizens were vaporized in an instant. Another 100,000 later died from injuries and the delayed effects of radiation.

Fat Man

Even then the Japanese Government refused to surrender. ‘Fat Man’, a plutonium bomb carried by the B29 “Bockscar”, was dropped on Nagasaki, on August 9.

The three cities originally considered for this second strike included Kokura, Kyoto and Niigata. Kyoto was withdrawn from consideration due to its religious significance. Niigata was taken out of consideration due to the distance involved.

Kokura was the primary target on this day, but local weather reduced visibility. Bockscar criss-crossed the city for the next 50 minutes, but the bombardier was unable to see well enough to make the drop. Japanese anti-aircraft fire became more intense with every run, and Second Lieutenant Jacob Beser reported activity on the Japanese fighter direction radio bands.

In the end, 393rd Bombardment Squadron Commander Major Charles Sweeney bypassed the city and chose the secondary target, the major shipbuilding center and military port city of Nagasaki.

The 10,000lb, 10𔄂″ weapon was released at 28,900′. 43 seconds later at an altitude of 1,650′, a perfect circle of 64 detonators exploded inside the heart of the bomb, compacting the plutonium core into a supercritical mass which exploded with the force of 20,000 tons of high explosive.

In the early 1960s, the Nagasaki Prefectural Office put the death count resulting from this day, at 87,000. 70% of the city’s industrial zone was destroyed.

Japan surrendered unconditionally on the 14 th of August, ending the most destructive war in history.

Nazi Germany was, in fact, working on a nuclear weapon, and had begun before the allies. They chose to pursue nuclear fusion, colliding atomic particles together to form a new type of nuclear material, instead of fission, the splitting of the atom which resulted in the atomic bomb.

That one critical decision, probably taken in some laboratory or conference room, put Nazi Germany behind in the nuclear arms race. How different would the world be today, had Little Boy and Fat Man had swastikas painted on their sides.

Letter to Truman about the Manhattan Project

After students have answered all of the analysis questions, ask them to share their answers and brainstorm the answer to the question: What is the Highly Secret Matter? Make sure that they point out specific evidence from the document to support their answers before revealing the correct response.

Prompt students to reflect on the fact that this project was so secret that the new President did not know about it. Ask them if they think that the Vice President (Truman before he became President) should have known about it, and what they think it means that so few people knew. Discuss the role of the commander in chief and the important — and ultimately controversial — decision Truman faced over whether to use nuclear weapons to end WWII.

Documents in this activity

To the extent possible under law, National Archives Education Team has waived all copyright and related or neighboring rights to " Letter to Truman about the Manhattan Project ".

This letter was sent to President Harry Truman by some scientists involved in the Manhattan Project following the end of the war in Europe. In the letter, the scientists outline the responsibility and concerns regarding the United States having and using atomic technology as offensive military action.

  • How did the scientists describe the responsibility the U.S. considering atomic weapons?
  • What conditions did the scientists feel needed to be present to consider dropping the atomic bomb? Were those conditions met?

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.

51f. The Manhattan Project

This once classified photograph features the first atomic bomb &mdash a weapon that atomic scientists had nicknamed "Gadget." The nuclear age began on July 16, 1945, when it was detonated in the New Mexico desert.

Early in 1939, the world's scientific community discovered that German physicists had learned the secrets of splitting a uranium atom. Fears soon spread over the possibility of Nazi scientists utilizing that energy to produce a bomb capable of unspeakable destruction.

Scientists Albert Einstein , who fled Nazi persecution, and Enrico Fermi , who escaped Fascist Italy, were now living in the United States. They agreed that the President must be informed of the dangers of atomic technology in the hands of the Axis powers. Fermi traveled to Washington in March to express his concerns to government officials. But few shared his uneasiness.

Leaving nothing to chance, Los Alamos atomic scientists conducted a pre-test test in May 1945 to check the monitoring instruments. A 100-ton bomb was exploded some 800 yards from the Trinity site where Gadget would be detonated a few weeks later.

Einstein penned a letter to President Roosevelt urging the development of an atomic research program later that year. Roosevelt saw neither the necessity nor the utility for such a project, but agreed to proceed slowly. In late 1941, the American effort to design and build an atomic bomb received its code name &mdash the Manhattan Project .

At first the research was based at only a few universities &mdash Columbia University, the University of Chicago and the University of California at Berkeley. A breakthrough occurred in December 1942 when Fermi led a group of physicists to produce the first controlled nuclear chain reaction under the grandstands of Stagg Field at the University of Chicago.

Enrico Fermi, a physicist who left fascist Italy for America, encouraged the U.S. to begin atomic research. The result was the top-secret "Manhattan Project."

After this milestone, funds were allocated more freely, and the project advanced at breakneck speed. Nuclear facilities were built at Oak Ridge, Tennessee and Hanford, Washington. The main assembly plant was built at Los Alamos, New Mexico . Robert Oppenheimer was put in charge of putting the pieces together at Los Alamos. After the final bill was tallied, nearly $2 billion had been spent on research and development of the atomic bomb. The Manhattan Project employed over 120,000 Americans.

Secrecy was paramount. Neither the Germans nor the Japanese could learn of the project. Roosevelt and Churchill also agreed that Stalin would be kept in the dark. Consequently, there was no public awareness or debate. Keeping 120,000 people quiet would be impossible therefore only a small privileged cadre of inner scientists and officials knew about the atomic bomb's development. In fact, Vice-President Truman had never heard of the Manhattan Project until he became President Truman.

Although the Axis powers remained unaware of the efforts at Los Alamos, American leaders later learned that a Soviet spy named Klaus Fuchs had penetrated the inner circle of scientists.

This crater in the Nevada desert was created by a 104 kiloton nuclear bomb buried 635 feet beneath the surface. It is the result of a 1962 test investigating whether nuclear weapons could be used to excavate canals and harbors.

By the summer of 1945, Oppenheimer was ready to test the first bomb. On July 16, 1945, at Trinity Site near Alamogordo, New Mexico , scientists of the Manhattan Project readied themselves to watch the detonation of the world's first atomic bomb. The device was affixed to a 100-foot tower and discharged just before dawn. No one was properly prepared for the result.

A blinding flash visible for 200 miles lit up the morning sky. A mushroom cloud reached 40,000 feet, blowing out windows of civilian homes up to 100 miles away. When the cloud returned to earth it created a half-mile wide crater metamorphosing sand into glass. A bogus cover-up story was quickly released, explaining that a huge ammunition dump had just exploded in the desert. Soon word reached President Truman in Potsdam, Germany that the project was successful.

On this day, FDR approves funding the Manhattan Project

On this day in 1941, President Franklin D. Roosevelt orders Dr. Vannevar Bush to move forward with a top-secret project that led to the world's first atomic bombs. Over the following four years, the Manhattan Project was shrouded in secrecy, despite more than 100,000 people working on it.

At that key meeting in Washington was Henry Wallace, then the Vice President, who had been briefed in July 1941 about discussions that started in 1939 after FDR received a letter from Albert Einstein and Leo Szilard about the potential of nuclear research's military use. The project's secrecy was so great that Harry Truman, who replaced Wallace as Vice President in 1945, had to be briefed on it after he became President after Roosevelt's death.

According to accounts of the meeting, President Roosevelt told Bush to find out how much it would cost to build the bomb, if it could be built, including the costs of construction facilities. Bush also updated Roosevelt on the work of British and American scientists. The meeting was one of several decisions Roosevelt made in secret about the bomb in that time peiod.

At the October 9 meeting, Roosevelt also indicated he would find funding for the project, which became much simpler after the United States officially entered World War II two months later. In January 1942, President Roosevelt officially approved the project to make the first atomic weapons.

General Leslie Groves was put in charge of the massive effort, which included the federal government seizing land in Oak Ridge, Tennessee to build a production facility housing about 75,000 workers. Within the government, only a handful of people knew the totality of the Manhattan Project, including a core group within the Executive Branch.

After the war, Groves disclosed that some people in Congress also knew the project's importance, such as the leaders of the House and Senate. They were able to get funding for the Manhattan Project buried inside appropriations bills as needed, telling other members they were part of the war effort and beyond questioning. One Senator who had questions about the unknown project's expenses was then-Senator Harry Truman, who was persuaded to drop his investigation by the Secretary of War, Henry Stimson.

In all, the United States spent an estimated $2 billion on a project that employed more than 120,000 people to build a nuclear weapon. The critical field test of a nuclear device was scheduled for July 16, 1945, to coincide with the arrival of President Truman at the Potsdam Conference.

The test bomb was placed on a 100-foot tower near Alamogordo, New Mexico. Government officials and scientists weren&rsquot sure what to expect when the device called Gadget was detonated at 5:30 a.m. The explosion was about four times stronger than anticipated by some scientists at Los Alamos Robert Oppenheimer later remarked the blast reminded him of a passage from Hindu text, the Bhagavad-Gita: "Now I am become Death, the destroyer of worlds."

Within a month, the United States military had dropped two atomic bombs on Japan, facilitating the end of World War II and the start of the Cold War. Truman had told Soviet leader Joseph Stalin about a new weapon of mass destruction while at Potsdam the Soviets already knew secretly about the American program and were working on their own atomic research.

Dawn of the Atomic Age

This site tells the story about the people, events, science, and engineering that led to the creation of the atomic bomb, which helped end World War II.

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A secret factory that made plutonium.

Los Alamos

A secret lab that built the bombs.

Oak Ridge

A hidden complex that enriched uranium.

Contact the Park

Mailing Address:

Manhattan Project National Historical Park
c/o NPS Intermountain Regional Office
P.O. Box 25287
Denver , CO 80225-0287


(505) 661-6277
This phone number is for the Los Alamos Unit Visitor Center. You may also contact the Oak Ridge Unit Visitor Center at (865) 482-1942, or the Hanford Unit Visitor Center at (509) 376-1647.