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Truman and the Atomic Bomb

Truman and the Atomic Bomb

Author: Sophia Tutorial

Understand the motivations behind the decision to drop the atomic bombs on Japan

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what's covered
All of the major powers in World War II developed powerful weapons. The most powerful weapon produced during the war was the atomic bomb.

President Harry Truman’s decision to drop the atomic bomb on Japan in August of 1945 is one of the most controversial in world history. In examining primary sources, historians have uncovered a number of factors that influenced this decision.

  1. The Manhattan Project
  2. Deciding to Drop the Atomic Bomb
  3. Dropping the Bomb
  4. The Consequences of the Bomb

1. The Manhattan Project

German scientists learned how to split uranium atoms in 1939. This discovery led to the creation of the atomic bomb. Albert Einstein, who immigrated to the United States in 1933 to escape the Nazis, urged President Roosevelt to launch an atomic research project. Roosevelt agreed, and in late 1941 the program received its code name: the Manhattan Project.

term to know
Manhattan Project
The code name given to the research project that developed the atomic bomb

The possibility that Germany might develop an atomic bomb first influenced Roosevelt’s decision to begin the Manhattan Project. However, German scientists had stopped trying to develop an atomic bomb because Germany lacked the resources to do so while fighting a two-front war against the Allies. The U.S. was unaware that the Germans had abandoned their bomb-development project.

The Manhattan Project, on the other hand, had plenty of support: money, manpower, and resources.

did you know
The Manhattan Project ultimately employed 150,000 people and cost $2 billion.

The project required machinery, plants, and material that were only available in the United States. The project also benefited from public works projects constructed by New Deal work relief agencies.


A uranium-extraction facility at Oak Ridge, Tennessee, used electricity and infrastructure developed by the Tennessee Valley Authority (TVA). The TVA was a New Deal program that built dams on the Tennessee River to provide hydroelectric power to the region.

A photograph of the uranium enrichment facility at Oak Ridge, Tennessee, in 1945.

Scientists who worked on the Manhattan Project were supported by federal funding and infrastructure. The fact that the U.S. had not experienced the wartime devastation and destruction that impacted Europe and Asia enabled the project to advance quickly.

Scientists at Los Alamos, New Mexico, successfully tested an atomic device in July of 1945. The U.S. had invested a significant amount of money and manpower into development of this weapon. Now the President would decide whether to use it against an enemy.

2. Deciding to Drop the Atomic Bomb

Rather than assessing a single decision to drop the atomic bomb, historians often examine the series of decisions that led to the use of the bomb. A number of factors were considered by the President and his advisors before they decided to use the weapon.

Harry Truman knew nothing about the Manhattan Project until he became President following the death of Franklin Roosevelt in April of 1945.

On April 24, 1945, Secretary of War Henry Stimson wrote to President Truman, “I think it is very important that I should have a talk with you as soon as possible on a highly secret matter.” Shortly thereafter, Truman learned about the Manhattan Project.

Truman may not have comprehended the destructive capabilities of the atomic bomb. Some of the scientists who built the bomb were surprised by its power, and expressed concern about its military use.

On July 17, 1945 — one day after the first successful test of the atomic bomb in New Mexico — a small group of project scientists sent a petition to President Truman. They asked him to consider the “moral responsibilities” when deciding whether to use the atomic bomb. Here is an excerpt from their petition:

Petition to President Truman from Manhattan Project Scientists

“The development of atomic power will provide the nations with new means of destruction. The atomic bombs at our disposal represent only the first step in this direction, and there is almost no limit to the destructive power which will become available in the course of their future development. Thus a nation which sets the precedent of using these newly liberated forces of nature for purposes of destruction may have to bear the responsibility of opening the door to an era of devastation on an unimaginable scale.”
think about it
According to this group of scientists, what responsibilities will President Truman bear if he decides to drop the atomic bomb?

Despite these concerns regarding the military uses of atomic power, the use of the atomic bomb as a way to end World War II gained significance during the summer of 1945, as the war against Japan continued.

In deciding whether to drop the bomb, Truman and his advisors considered the experience of the U.S. armed forces in fighting the Japanese, particularly the atrocities committed by Japanese soldiers against captured Americans.


Following the surrender of American and Filipino forces in the Philippines in early 1942, approximately 650 American soldiers lost their lives during the “Bataan Death March”. Thousands more died from disease and starvation in Japanese prisoner-of-war camps.

The fact that Japanese soldiers and civilians often refused to surrender to U.S. forces as they advanced towards Japan factored into deliberations on the use of the atomic bomb. During the Battle of Saipan (1944), several thousand Japanese soldiers and civilians committed suicide by jumping off a cliff rather than surrender to the Americans.

Towards the end of the war, almost 4,000 Japanese pilots died in kamikaze, or suicide, attacks against U.S. ships. Americans saw the attacks as proof of the irrationality of Japanese martial values and mindless loyalty to the Japanese emperor.

U.S. military officials knew that many Japanese soldiers and civilians would die rather than surrender when the United States invaded Japan. War planners estimated that as many as 250,000 American soldiers would die in the invasion — one more factor that Truman took into account while considering his decision.

The war in the Pacific intensified racist, anti-Japanese sentiment in the United States, as displayed by wartime propaganda.

Propaganda often portrayed the Japanese as inhuman and inferior (a). In the aftermath of the attack on Pearl Harbor, Japanese soldiers (b) were depicted as uncivilized and barbaric.

think about it
What kinds of reactions were anti-Japanese posters like those above designed to elicit from American audiences?

Historians have focused on two additional factors that impacted the decision to drop the atomic bomb: concerns about the Soviet Union, and the bombing of civilians during previous wars.

  • Concerns about the Soviet Union

When the war in Europe ended, the Soviet Union agreed to assist the U.S. in the war against Japan. Some American officials worried that the Soviets would occupy — and seize — territory as they fought the Japanese in Asia.

Historians have found evidence that James F. Byrnes, the Secretary of State (1945-1947), urged Truman to drop the atomic bomb on Japan as soon as possible — to end the war, and to prevent Soviet expansion in Asia. Based on this evidence, some historians argue that the United States dropped atomic bombs on Japan as a warning to the Soviet Union, thus setting the stage for the Cold War.

term to know
Cold War
The prolonged period of tension between the United States and the Soviet Union following World War II
  • Prior bombings of civilian populations during wartime

By 1945, some war planners no longer considered moral questions regarding the bombing of civilians. In Europe, Nazi Germany targeted civilian populations when it bombed Great Britain. The British and, later, the Americans, returned the favor during air raids on Germany.


In February of 1945, a joint Anglo-American bombing raid killed 35,000 civilians in the German city of Dresden.

As American forces drew closer to Japan, the U.S. Air Force bombed Tokyo and other Japanese cities. Admiral William Halsey spoke for many Americans when he urged those under his command to, “Kill Japs! Kill Japs! Kill more Japs!”

The U.S. and its allies sought the unconditional surrender of the Axis Powers. This required defeat of the civilian population, as well as the armed forces. The bombing of towns and cities destroyed vital infrastructure and lowered civilian morale. Both were necessary to secure victory.

3. Dropping the Bomb

All of the factors noted above influenced President Truman’s decision to drop the atomic bomb. Beginning in the spring and summer of 1945, military officials selected potential targets.

did you know
Suspecting that the immediate bomb blast would extend over one mile, and knowing that the bomb would start fires, planners looked for a compact city of significant military value with wood-frame buildings built close together.

Hiroshima, headquarters of the Japanese Second Army, and the communications and supply hub for southern Japan, was chosen as the first target. At 8:15AM on Monday, August 6, 1945, the Enola Gay, a B-29 named after its pilot’s mother, dropped an atomic bomb named “Little Boy” on the city.

Survivors, who were eating breakfast or preparing for school at the time, recalled seeing a bright light and then being blown across the room. The intense heat of the blast melted stone and metal, and ignited fires throughout the city. One survivor remembered watching his mother and brother burn to death as fire consumed their home. Within an hour of the bombing, radioactive “black rain” began to fall on the city.

Hiroshima lay in ruins after the dropping of the atomic bomb on August 6, 1945.

did you know
During the initial blast, two-thirds of the buildings in Hiroshima were destroyed and approximately 70,000 people died. The same number would later die of radiation poisoning.

Later that day, President Truman released a statement informing the American people of what had occurred. Here is an excerpt from that statement:

President Truman, on the Bombing of Hiroshima

“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….

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.”
think about it
  1. According to Truman’s statement, why did the U.S. drop the atomic bomb on Hiroshima?
  2. Given the evidence provided in this tutorial, do you think most Americans at the time would have agreed or disagreed with Truman’s decision?

When Japan refused to surrender after the bombing of Hiroshima, the U.S. dropped a second atomic bomb on Nagasaki, a major industrial center and the largest seaport in southern Japan.

A photograph of the “mushroom cloud” created by the atomic blast over Nagasaki on August 9, 1945.

At least 60,000 people were killed during the initial blast at Nagasaki on August 9, 1945.

The dropping of both bombs forced Japan to surrender. During the late stages of the war, members of Japan’s Imperial Council hoped to negotiate peace with the United States. Following the destruction of Nagasaki, however, Emperor Hirohito agreed to the U.S. demand that Japan surrender unconditionally. Although surrender was hateful to many Japanese, Japan’s military forces had been defeated and bombing had reduced its cities and industries to rubble. The future of Japan looked bleak. Reconstruction, under the oversight of U.S. occupation forces, would be an enormous job.

4. The Consequences of the Bomb

Although the atomic bombs forced Japan to surrender, some Americans worried about other consequences.

After the the second bomb was dropped, Samuel Cavert, secretary of the Federal Council of the Churches of Christ in America, sent a telegram to President Truman stating that “many Christians” were “deeply disturbed” by the use of atomic bombs.

Truman’s reply, in which he indicated that war sometimes required ruthless action, was blunt:

President Truman, on the Use of Atomic Bombs in Japan

“Nobody is more disturbed over the use of Atomic bombs than I am but I was greatly disturbed over the unwarranted attack by the Japanese on Pearl Harbor and their murder of our prisoners of war. The only language they seem to understand is the one we have been using to bombard them.

When you have to deal with a beast you have to treat him as a beast. It is most regrettable but nevertheless true.”
think about it
  1. According to the evidence provided above, why did President Truman decide to drop the atomic bombs on Japan?
  2. Does Truman’s reference to the Japanese as “a beast” indicate that racism played a role in the conduct of the war against Japan?

The Manhattan Project exemplified America’s ability to mobilize and produce the weapons necessary to defeat the Axis Powers during World War II. Following the war, the U.S. would never leave the global stage. Its early mastery of atomic power made the U.S. the dominant force in the postwar world.

Given the factors involved in the decision to drop the atomic bomb provided in this tutorial, would the U.S. have dropped an atomic bomb on Germany if the opportunity had presented itself?
Instead of viewing the dropping of the atomic bomb on Japan as the result of a single decision, it is best to examine the factors that President Truman and his advisors considered before making the decision. Examination of primary sources indicates that the interplay between these factors led to Truman’s decision to drop atomic bombs on Hiroshima and Nagasaki.

This tutorial curated and/or authored by Matthew Pearce, Ph.D

Source: Uranium, PD, Letter Stimson/Truman, 4/24/45, Truman Library, Petition to the President of the US, 7/17/45, Truman Library White House Press Release, 8/6/45, Truman Library Hiroshima after dropping of bomb, 8/6/45, Truman Library, Atomic blast over Nagasaki, 8/9/45, Truman Library, Correspondence between Truman & Cavert, 8/11/45, Truman Library, Openstax tutorial 27.3 & 27.4 .Some sections edited

Terms to Know
Cold War

the prolonged period of tension between the United States and the Soviet Union following World War II.

Manhattan Project

the code name given to the research project that developed the atomic bomb in the United States.

People to Know
Albert Einstein

German scientist and immigrant to the U.S. who urged President Franklin Roosevelt to launch the American atomic research project that eventually became the Manhattan Project.

Harry S. Truman

democratic U.S president who took office after Franklin Roosevelt’s death in 1945 and remained in office until 1953; navigated the decision to drop the atomic bomb on Japan and the first years of the Cold War between the U.S. and the Soviet Union.