Beginning in 1942, the US Army operated a program called the Manhattan Project that focused on the research and development of a new type of weapon, the atomic bomb. On August 6, 1945, the US military dropped the first atomic bomb used in warfare on the Japanese city of Hiroshima, in the hopes of forcing a Japanese surrender and bringing a swift end to World War II. The blast leveled part of the city and resulted in some eighty thousand deaths, approximately 30 percent of the city’s population, with additional casualties caused later due to the effects of radiation exposure. However, the Japanese did not immediately surrender, and three days later the United States dropped a second atomic bomb on the city of Nagasaki, causing nearly forty thousand deaths. The Japanese surrendered soon after, and the United States led the Allied occupation and reconstruction of the devastated country. As part of its work, the US Army undertook a scientific study of the lasting effects of the atomic blasts in Hiroshima and Nagasaki. Their report detailed the efficacy and immense destruction of these attacks.

Beginning in 1942, the US Army operated a program called the Manhattan Project that focused on the research and development of a new type of weapon, the atomic bomb. On August 6, 1945, the US military dropped the first atomic bomb used in warfare on the Japanese city of Hiroshima, in the hopes of forcing a Japanese surrender and bringing a swift end to World War II. The blast leveled part of the city and resulted in some eighty thousand deaths, approximately 30 percent of the city’s population, with additional casualties caused later due to the effects of radiation exposure. However, the Japanese did not immediately surrender, and three days later the United States dropped a second atomic bomb on the city of Nagasaki, causing nearly forty thousand deaths. The Japanese surrendered soon after, and the United States led the Allied occupation and reconstruction of the devastated country. As part of its work, the US Army undertook a scientific study of the lasting effects of the atomic blasts in Hiroshima and Nagasaki. Their report detailed the efficacy and immense destruction of these attacks.

The study of physics advanced greatly during the early twentieth century. Scientists such as Albert Einstein, Otto Hahn, and Fritz Strassmann promulgated new theories that increased the depth of human knowledge about how the tiny building blocks of all matter, atoms, worked together to create different substances. These advances, coupled with the political and military tensions that sparked World War II in 1939, encouraged a growing interest in research efforts to harness physical forces to create potentially devastating new weapons. At the urging of Einstein, US president Franklin D. Roosevelt authorized research into possible military applications of nuclear power in 1939. By mid-1942, this research had been formalized under the command of the US Army Corps of Engineers and dubbed the Manhattan Project.

Researchers in the Manhattan Project overcame a series of technological challenges to determine the process needed to cause the bonds holding atoms together to break, which releases the force that normally keeps matter together in an uncontrollably explosive fashion. Their work culminated in the first successful atomic bomb test on July 16, 1945. On July 26, 1945, the United States, the United Kingdom, and China issued the Potsdam Declaration, threatening Japan’s “utter destruction” if the country did not accept the Allied terms of surrender.

On August 6, 1945, the US military dropped “Little Boy,” the first atomic bomb used in warfare, on the Japanese city of Hiroshima. President Harry S. Truman, who had succeeded Roosevelt that spring, hoped that this show of military might would force a Japanese surrender and end World War II without requiring the US military to launch what was certain to be a long and bloody invasion of the Japanese main islands. The force released by the detonation of the atomic bomb proved immensely destructive. But it did not have the desired political result, as the Japanese government did not issue an immediate unconditional surrender. Over the next few days, however, the Japanese position worsened. The Soviet Union, which had not yet declared war on Japan and had concentrated its military along the European Eastern Front, issued a declaration of war against Japan on August 9 and began an invasion of the Japanese-controlled region of Manchuria in mainland China. Later that day, the US military dropped a second atomic bomb on the city of Nagasaki.

Japanese leaders concluded that the situation was hopeless. On August 10, the Japanese government agreed to end hostilities and accept the Allied terms of surrender that had been laid out in the Potsdam Declaration a few weeks previously, and the surrender of Japan was formalized on September 2. The terms of surrender stipulated the occupation of Japanese territory by Allied forces with the purpose of thoroughly demilitarizing Japan and providing support for its rebuilding as a friendly nation. Under this provision, the US military had the authority to conduct thorough investigations into the sites of the two atomic bombings, and these took place in September 1945.

Research and development of the atomic bomb took place under the official command of the United States Army Corp of Engineer’s Manhattan District, which lent the program its popular name, the Manhattan Project. At the time this report was compiled, Major General Leslie Groves had served as head of the Manhattan Project for about four years. In this capacity, he had overseen the teams responsible for the research and construction of the bomb and led efforts to determine when and how it could be used. Numerous scientific, engineering, and medical groups from the United States and Great Britain helped develop the report, including the Special Manhattan Engineer District Investigating Group, the United States Strategic Bombing Survey, the British Mission to Japan, and the Joint Atomic Bomb Investigating Group. On the scientific side, researchers such as Robert Serber, Hans Bethe, and other nuclear physicists worked on the atomic bomb at the Manhattan Project’s outpost at the Los Alamos National Laboratory in New Mexico. Experts in medicine and engineering contributed to portions of the project dealing with their respective fields.

The following are the main conclusions which were reached after thorough examination of the effects of the bombs dropped on Hiroshima and Nagasaki:

1. No harmful amounts of persistent radioactivity were present after the explosions as determined by:

A. Measurements of the intensity of radioactivity at the time of the investigation; and

B. Failure to find any clinical evidence of persons harmed by persistent radioactivity.

The effects of the atomic bombs on human beings were of three main types:

A. Burns, remarkable for (1) the great ground area over which they were inflicted and (2) the prevalence of “flash” burns caused by the instantaneous heat radiation.

B. Mechanical injuries, also remarkable for the wide area in which suffered.

C. Effects resulting from penetrating gamma radiation. The effects from radiation were due to instantaneous discharge of radiation at the moment of explosion and not to persistent radioactivity (of either fission products or other substances whose radioactivity might have been induced by proximity to the explosions).

The effects of the atomic bombs on structures and installations were of two types:

A. Destruction caused by the great pressure from the blast; and

B. Destruction caused by the fires, either started directly by the great heat radiation, or indirectly through the collapse of buildings, wiring, etc.

4. The actual tonnage of T.N.T. which would have caused the same blast damage was approximately of the order of 20,000 tons.

5. In respect to their height of burst, the bombs performed exactly according to design.

6. The bombs were placed in such positions that they could not have done more damage from any alternative bursting point in either city.

7. The heights of burst were correctly chosen having regard to the type of destruction it was desired to cause.

8. The information collected would enable a reasonably accurate prediction to be made of the blast damage likely to be caused in any city where an atomic explosion could be effected.

fission: the act of cleaving or splitting into parts; in physics, called nuclear fission, the splitting of the nucleus of an atom into nuclei of lighter atoms, accompanied by the release of energy

tonnage: capacity or weight in tons

Because the bombings at Hiroshima and Nagasaki were the first—and, to date, the only—uses of atomic weaponry in warfare, neither scientists nor military personnel were entirely certain what the bombs’ effects would be on their targets. This section of the US Army’s report, therefore, summarizes the findings of site researchers in three key areas: long-term radiation levels, the immediate and short-term effects on humans, and the effects of the bombings on physical property. In all cases, these effects were found to be significant upon detonation, with a lessening of effects over time.

This section of the report details the findings regarding levels radioactivity. Measurements taken short weeks after the bombings showed “no harmful amounts of persistent radioactivity” based on direct measurements and observations of people in the cities. Because both bombs were detonated in the air over populated areas, the ground absorbed relatively little radiation. As atomic particles decayed, they lost their radioactive properties and became harmless. By the time researchers came to Hiroshima and Nagasaki, therefore, levels of radioactivity had declined to harmless levels.

Next, the report summarizes the effects of the bombings on individuals and structures in the affected areas. It identifies three main effects on humans: flash burns, physical or “mechanical” injuries caused by falling buildings or debris, and exposure to high levels of radioactivity at the time of detonation. Flash burns occur when tissue is exposed to sudden intense heat. Many thousands of people in the affected areas suffered intense burns from the heat generated by the atomic bombs. The blasts also set numerous structural and ground fires, and the bombs’ force knocked down walls and roofs; the report concludes that devastation was so great that the bombs “could not have done more damage from any alternative bursting point in either city.” Less traditional was the damage caused by exposure to a blast of high-level radiation. Some people in both cities received sufficient doses of radioactivity to bring on radiation sickness, which can cause problems ranging from nausea to death, depending on the amount of radiation a person’s body absorbed. Symptoms could begin minutes to several weeks after exposure.

The report concludes by asserting that the bombings were, from a military standpoint, a success. The bombs performed as intended by causing immense destruction, and the military believed that this outcome could be achieved again in a different location if desired.

The determinations of the report reflect two significant conclusions. First, the report affirmed the devastating power of the atomic bomb as a tool of war and destruction. Equally, it showed that the military had effectively and purposefully harnessed this immense force—and stated that they would be willing to use it again if warranted. Control of the bomb and the threat of its usage informed American identity and policy for decades to come as the nation engaged in the Cold War, a geopolitical struggle for supremacy with the Soviet Union.

As World War II came to a close, competition for international influence and power increased between the ideologically opposed United States and Soviet Union, who had united in their opposition to the Nazi German threat during the conflict. However, as the reorganization of Eastern Europe began, the Soviet Union broke away from the remainder of the Allies in asserting its interests. By the end of the decade, the United States under the Truman Doctrine had the stated policy of giving aid to those who resisted the spread of Communism and Soviet influence around the globe. At the same time, the Soviet Union successfully developed its own nuclear weapons and, in time, the advanced aeronautical technology needed to launch them over long distances to US territory. During the 1950s, Americans widely feared that they could be victims of the same kind of devastating attack that the United States had launched against Hiroshima and Nagasaki. In the early 1960s, this possibility seemed increasingly likely when tensions over the placement of Soviet missile bases on the communist island of Cuba, just ninety miles from the United States, increased so greatly that nuclear attack seemed imminent. Although the Cuban missile crisis was resolved diplomatically, the nuclear arms race continued into the 1980s.

However, neither the United States nor any other power have used a nuclear device in warfare since the bombings of Hiroshima and Nagasaki. Truman, who had authorized the bombing of Hiroshima and Nagasaki, declined to do so again during the Korean War later in the 1940s and early 1950s. Presidents who oversaw rapid nuclear expansion equally refused to actually use the weapons. The force shown in the Japanese attacks proved too devastating to make it a desirable weapon except as a very last resort.

• Hersey, John.Hiroshima. New York: Knopf, 1946. Print.
• Hogan, Michael J., ed.Hiroshima in History and Memory. New York: Cambridge UP, 1996. Print.
• Kort, Michael.Columbia Guide to Hiroshima and the Bomb. New York: Columbia UP, 2007. Print.