The 1945 US Use of Atomic Bombs

Background and Reasons for Hiroshima and Nagasaki

Last October, I had a two-day business trip to Hiroshima to write a thesis on the atomic bombing. During my stay, I visited the Hiroshima Peace Memorial Museum and would like to explain why atomic bombs were dropped on Hiroshima and Nagasaki.

Ruin of Hiroshima Prefectural IndustrialPromotion Hall — Ruin of the Hiroshima Prefectural Industrial Promotion Hall, preserved as a memorial for peace. Photographed in October 2024.

Despite being October, the lingering summer heat in Japan meant it was hot during the day, reaching up to 25 degrees Celsius. Hiroshima’s basin topography, surrounded by mountains, also made it humid. Nevertheless, unlike other Japanese cities, I was impressed by the spacious interiors of cafes and restaurants.

So, this time, the question is: why did the United States deliberately drop atomic bombs on Hiroshima and Nagasaki in August 1945? Was it to end the war quickly? Or to prevent greater American casualties?

However, I believe there were other reasons. As explained in the previous installment, I discussed why Japan did not surrender even after Germany’s surrender in May 1945.

Even within Japan’s Liberal Democratic Party today, factional politics are divided in various ways. The same was true during World War II. Outwardly, the Emperor appeared to be the supreme leader, with the Prime Minister below him. However, in reality, there was a military faction that did not listen to the directives of the Emperor or the Prime Minister.

The Hidden Truth Behind Japan’s Delayed Surrender Until August 1945
Japan’s delayed surrender was the result of the country’s unique modern political structure. The power struggle surrounding the symbolic figure of the…
Link to previous post

Ultimately, Japan was hit by two atomic bombs in Nagasaki and Hiroshima during World War II. But why Hiroshima and Nagasaki in particular? And why use atomic bombs at all? International politics is determined by various reasons, but the important thing is that dropping atomic bombs on Japan was obviously advantageous for the United States, so they did it. What benefits did the United States see in dropping atomic bombs in August 1945?

Contents

The US Perspective at the Time

President Truman, who decided to use atomic bombs during World War II, was unaware of the Manhattan Project, which was underway in the United States, during his time as Vice President. This shows how secretly nuclear weapons were being developed. Of course, nuclear weapons were not yet completed at the time, but Truman learned about the US nuclear weapons development shortly after becoming President. However, Germany had already surrendered in May 1945, before the nuclear weapons were even developed. Japan was the only remaining country that had not surrendered. The US Department of Defense began analyzing how to make Japan surrender. The analysis concluded that if US forces were to land on the Japanese mainland, US casualties alone were estimated to be as high as 30,000.

The analysis also suggested that if the Soviet Union, geographically close to Japan, were to attack, Japan might surrender very quickly, ending the war.

However, the international political and diplomatic officials in the Truman administration strongly opposed Soviet intervention in Japan’s surrender. They argued that even if US casualties were low due to Soviet intervention, the Soviet Union would gain influence in East Asia alongside the United States.

This illustrates that in war, the primary consideration is not just minimizing friendly casualties but also taking into account international political, diplomatic, and post-war arrangements.

US Goals

The US goal was set: to make Japan surrender without Soviet assistance, and with the fewest possible US casualties. In fact, this was in May 1945, two months before the nuclear weapons development was completed in July 1945. At the time, there was a belief that continued large-scale air raids on Tokyo would eventually lead to Japan’s surrender.

However, the crucial point was that the United States needed to make Japan surrender quickly, before the Soviet Union intervened. While making Japan surrender was important, what was even more important was to end the war in a way that allowed the United States to have sole influence in Northeast Asia in the post-war period, rather than sharing it with the Soviet Union.

Three Options

At the time, US government officials presented three options:

Resolve through peace negotiations with Japan: The key was for the US to gain influence and benefits in Northeast Asia after the war. However, this option was not adopted due to anti-Japanese sentiment within the United States.
Soviet intervention: Under President Roosevelt, there was an agreement between the US and the Soviet Union to jointly attack and resolve the situation with Japan. However, fortunately or unfortunately, when the US attacked Japan, the Soviet Union was focused on the communization of Europe after the war and did not attack Japan to advance into Northeast Asia.
Total mobilization and occupation by US Army and Navy (landing on the Japanese mainland): Inevitably, an option to land on the Japanese mainland emerged within the US government.

However, in July 1945, the United States successfully tested the atomic bomb through the Manhattan Project.

Reasons for Choosing Hiroshima and Nagasaki

The United States’ successful nuclear weapon test changed the situation significantly from when the three options were discussed. President Harry Truman recognized the potential of this weapon. The atomic bomb was seen as a powerful means to end the war quickly.

Having succeeded in the first nuclear weapon test in human history, the US then faced the question of where to drop this weapon. From May 1945, the Target Committee of the Manhattan Project (scientists and officers belonging to the Manhattan Project) selected potential atomic bombing sites, applying the following criteria:

Large cities with a diameter of over 3 miles.
Flat terrain around the city to maximize the blast effect.
Cities that had not yet suffered large-scale bombing.
Areas with a large civilian population as well as military facilities.

Why was a city with a large civilian population included in the guidelines? The reason was that since the military faction was independently initiating the war without following the directives of the Emperor or the Prime Minister, it was necessary to stimulate public sentiment within Japan to push for a quick surrender. Otherwise, the Japanese military faction was expected to continue delaying surrender. Another important point was to secure surrender before Soviet intervention.

Hiroshima, Kitakyushu, Niigata, and Nagasaki were selected as primary target cities for the atomic bombings. The order was to be flexibly adjusted based on weather conditions, but if Japan did not surrender, all four were to be targeted. In fact, dozens of cities had already been designated as secondary and tertiary targets. Among the many candidates, Nagasaki and Hiroshima were chosen as suitable based on the above guidelines.

Thus, on August 6, 1945, Little Boy (the atomic bomb) was dropped on Hiroshima. However, just two days later, on August 8, 1945, the Soviet Union suddenly declared war against Japan. As the US had anticipated, the Soviet Union decided to enter the war to secure influence in Northeast Asia as soon as it saw the atomic bombing of Hiroshima.

The day after the Soviet Union’s declaration of war, on August 9, 1945, Fat Man was scheduled to be dropped on Kitakyushu, but due to weather conditions, it was dropped on the nearby city of Nagasaki. Japan then unconditionally surrendered.

Outcome

The United States minimized its casualties and, at the same time, secured its influence in the crucial post-war Northeast Asian region, containing the Soviet Union. Just before Soviet intervention, on August 6, 1945, the first atomic bomb, “Little Boy,” was dropped on Hiroshima, followed by “Fat Man” on Nagasaki on August 9. Immediately after these events, Japan accepted unconditional surrender on August 15, ending World War II.

Looking at the documents from that time, the use of atomic bombs was a complex decision involving not only military objectives but also international political and diplomatic considerations. It was part of the US strategy to minimize war damage while securing dominance in the post-war world order. Simultaneously, this event demonstrated the destructive power of nuclear weapons and their impact on humanity, marking the beginning of the Cold War, a silent war between the United States and the Soviet Union.