Was the United States Justified in Using the Atomic Bombs Against Japan?: A Detailed Summary

The atomic bombing of Japan by the United States at the end of World War II is one of the most controversial events in all of world history.  World War II ended in Europe by May of 1945 with the defeat of Nazi Germany by the Allied Powers, but the war did not officially end in the Pacific Theater until the atomic bombing of Japan in August of 1945.  The United States had developed its atomic weaponry through its secretive program called the Manhattan Project.

World War IIHow It Worked

FactsthatCrunch

The Manhattan Project was the secret United States program that built the world's first atomic bombs during World War II, bringing together scientists, engineers, and military planners at sites across the country.

1

Scientists and military leaders launched the program in 1942 after research showed that an atomic bomb was possible. The U.S. government kept the project secret by giving it a misleading name and hiding its true purpose from nearly everyone outside it.

2

The project ran work at several locations at once. Uranium was enriched at Oak Ridge, Tennessee, plutonium was produced at Hanford, Washington, and bomb design was led by physicist J. Robert Oppenheimer at Los Alamos, New Mexico.

3

Thousands of workers were hired for jobs at these sites, but most were told only what they needed to know for their specific task. Very few people outside the top leadership knew the full scope of what was being built.

4

On July 16th, 1945, the project successfully tested the first atomic device at the Trinity test site in the New Mexico desert, confirming the weapon worked before any decision was made to use it in combat.

5

President Harry S. Truman approved the use of the bombs against Japan. The project delivered two completed weapons, Little Boy and Fat Man, which were used against Hiroshima on August 6th, 1945, and Nagasaki on August 9th, 1945.

Source: The National Archives, UK© factsthatcrunch.comMore World War II Facts →

Atomic Bombing of Japan – Summary

The first atomic bomb dropped on Japan was codenamedGiven a secret name to hide the true identity or purpose of something. Little Boy and was detonatedExploded or set off, used here to describe the moment an atomic bomb went off over a city. over the city of Hiroshima on August 6th, 1945.  Hiroshima is a city on the largest island of Japan and had a population of about 350,000 at the time of the atomic bombing.  The atomic bomb was carried to its target by an American B-29 bomber called the Enola Gay after United States President, Harry S. Truman, approved its use against Japan.

The city of Nagasaki was located in the southwest of Japan and had approximately 240,000 people.  The atomic bomb used against Nagasaki was called the Fat Man and was delivered to the city on August 9th, 1945, by the American B-29 bomber Bockscar.

Japan’s surrender was announced on August 15th, and World War II in the Pacific was over.  The formal surrender of Japan would occur on September 2nd, 1945, aboard the United States battleship USS Missouri.  There, Japanese officials signed the official Instrument of SurrenderThe official document signed by Japanese officials on September 2nd, 1945, formally ending World War II..

The atomic bombing of Japan by the United States in 1945 is one of the most controversial events in all of world history and many have argued both in favor of using the weapon and against using it in order to end World War II.  Regardless, the atomic bombing is a significant event for its role in ending World War II and for the impact it would have on all future warfare.  Read Atomic Bombing of Japan for more detailed information on the two atomic bombs used against Japan at the end of World War II.

World War IIWhat Happened NextInteractive

FactsthatCrunch

What happened next?

On August 6th, 1945, the American B-29 bomber Enola Gay dropped the atomic bomb called Little Boy on the Japanese city of Hiroshima. Japan did not surrender right away.

Three futures. One is real. Tap your call.

Japan surrendered the next day, and the war ended on August 7th, 1945.Three days later, on August 9th, 1945, a second atomic bomb called Fat Man was dropped on Nagasaki.American troops began landing on the Japanese home islands within the week.

What actually happenedOn August 9th, 1945, the B-29 bomber Bockscar dropped the Fat Man bomb on Nagasaki. Japan announced its surrender on August 15th, 1945, and formally signed the Instrument of Surrender aboard the USS Missouri on September 2nd, 1945. No invasion of Japan ever took place.

Source: National Archives© factsthatcrunch.comMore World War II Facts →

Atomic Bombing of Japan – Reasons For

Some historians have argued that the atomic bombing of Japan at the end of World War II was necessary and justifiedHaving good enough reasons to show that an action was fair or morally acceptable..  There have been several key reasons developed to support this position, such as:

• It led to a quick end to World War II.
• It saved the lives of American soldiers.
• It potentially saved the lives of Japanese soldiers and civilians.
• It forced Japan to surrender, which it appeared unwilling to do.
• It was revenge for Japan’s attack at Pearl Harbor.
• It matched the brutality that Japan used during the war.
• Japan was warned about the potential of the atomic bombs.
• The program to create the bombs was too expensive for them not to be used.
• Bombs are a natural part of war.
• It was necessary as a display for the emerging Cold War.

The above is a general list of reasons in favor of the use of the atomic bombs.  Read Reasons in Favor of the Atomic Bombing of Japan for more detailed reasons on why the United States was justified in using the atomic bombs against Japan.

World War IIColorizedInteractive

FactsthatCrunch

Black & whiteColor

Images of the two atomic bombings of Japan in 1945. Left is the mushroom cloud of the atomic bombing of Hiroshima on August 6th. Right is the mushroom cloud of the atomic bombing of Nagasaki on August 9th. (Both images colorized by historycrunch.com)

Colorized by: historycrunch.com© factsthatcrunch.comMore World War II Facts →

Atomic Bombing of Japan – Reasons Against

Although some historians were in favor of the use of the atomic bombs, many others have argued that the atomic bombing of Japan at the end of World War II was not necessary and unjustified.  There have been several key reasons developed to support this position, such as:

• It was inhumaneCruel and causing extreme suffering, especially to people who cannot defend themselves..
• It caused too much destruction.
• It killed too many innocent people, including children.
• It was unnecessary as Japan was essentially defeated.
• Japan was seeking surrender.
• It was not universally supported in the United States.
• The United States could have done something else.
• The United States should have waited longer between the two bombs.
• It was used more to scare the Soviet Union than to defeat Japan.
• It led to the modern atomic age and the threat of nuclear warfare.

The above is a general list against the use of the atomic bombs in World War II. Read Reasons Against the Atomic Bombing of Japan for more detailed reasons on why the United States was not justified in using the atomic bombs against Japan.

World War IIQ&A

FactsthatCrunch

Q

Was Japan already close to surrender before the atomic bombs were dropped in World War II?

A

Historians debate this question seriously. Some argue that Japan was essentially defeated and seeking surrender before the bombs fell, pointing to Japan's heavy losses across the Pacific and back-channel peace contacts through the Soviet Union in mid-1945. Others argue that Japan's military leaders were still preparing to fight a full invasion of the home islands, making a quick surrender far from certain without the bombs.

Source: US National Archives© factsthatcrunch.comMore World War II Facts →

Key Vocabulary

Frequently Asked Questions

Cite This Article

To cite this article as a source, use one of the formats below.

MLA: Beck, Elias. “Was the United States Justified in Using the Atomic Bombs Against Japan?: A Detailed Summary.” HistoryCrunch, 2 October 2026, https://historycrunch.com/was-the-united-states-justified-in-using-the-atomic-bombs-against-japan/.

APA: Beck, E. (2026). Was the United States Justified in Using the Atomic Bombs Against Japan?: A Detailed Summary. HistoryCrunch. https://historycrunch.com/was-the-united-states-justified-in-using-the-atomic-bombs-against-japan/

Chicago: Beck, Elias. “Was the United States Justified in Using the Atomic Bombs Against Japan?: A Detailed Summary.” HistoryCrunch. October 2, 2026. https://historycrunch.com/was-the-united-states-justified-in-using-the-atomic-bombs-against-japan/

Sources

• The National WWII Museum
• Imperial War Museum