Manhattan Project

Manhattan Project
Manhattan Project

The Manhattan Project was a secret U.S. weapons program that applied nuclear technology to create the first atomic bombs. Although other nations, including Great Britain, Germany, the Soviet Union, and Japan, had modest nuclear research programs during World War II, only the United States had the scientific talent, industrial capability, and financial resources to successfully create, test, and eventually use the world's most powerful weapon of the time.

In December 1938 German scientists Otto Hahn, Fritz Strassmann, and Lise Mietner discovered that bombarding an atom of radioactive uranium with neutrons caused its nucleus to split, thereby releasing an enormous burst of energy. This process would come to be called nuclear fission.

The development opened up the possibility for further research into harnessing this energy to be new sources of power as well as the possibility of new, more destructive types of weapons. In the 1930s the scientific community involved in nuclear research was international in character and included contributions from both Europe and North America. By 1939 political tensions in Europe caused many scientists to congregate in the United States and Britain, including many émigrés from Germany and Italy.


Work on using nuclear fission for military applications began in Germany on April 29, 1939, when the Reich Ministry of Education convened a secret conference and created a new research program. Germany also banned the export of uranium, an essential and rare element needed for this research.

In 1939 Leo Szilard, a Hungarian émigré physicist, understood the military potential of nuclear fission and the danger if Germany harnessed this power. Szilard went to the United States to enlist the help of Albert Einstein, at that time the most famous scientist in the world.

In August 1939 Einstein wrote a letter to President Franklin D. Roosevelt warning that a weapon based on nuclear fission was possible and that Germany could be in the process of constructing such a weapon. Einstein further urged the president to begin a project to develop an atomic bomb. Roosevelt responded by creating a committee to study the military implications of nuclear physics.

Manhattan Project warning sign
Manhattan Project warning sign

In December 1941, after the Japanese attack on Pearl Harbor, the United States went to war with Germany, Japan, and Italy. During the war the fundamental military strategy of the United States was to achieve complete victory at the lowest cost to U.S. lives. U.S. officials believed that an atomic bomb could shorten the war and reduce the number of U.S. casualties.

By early 1942 British scientists concluded that a uranium weapon was feasible. Based on these reports the secret weapons program was put under the auspices of the U.S. War Department and was code-named the Manhattan Engineer District, more commonly known as the Manhattan Project, because it originally was to be headquartered in New York City.

In September 1942 army general Leslie Groves was named director. Groves soon appointed J. Robert Oppenheimer, a theoretical physicist from the University of California at Berkeley, scientific director.

The project soon encompassed a crew of over 100,000 people, involving 37 installations in 13 states, and more than a dozen university laboratories. Secrecy was considered to be of the utmost importance. In fact, many of the scientists and engineers were given only information that immediately affected their work, and they therefore were unaware of the larger implications of their research.

On December 2, 1942, a team led by Enrico Fermi, a Nobel Prize–winning physicist émigré from Italy, created the first controlled, self-sustaining nuclear chain reaction at the University of Chicago. This proved that an atomic bomb many times more powerful than conventional weapons was possible.

The project focused on two main tasks. The first was the design of the bomb. Most of this work was done at the Los Alamos weapons lab in New Mexico under the direct supervision of Oppenheimer, who supervised the actual design and construction of the bomb. The other task, the production of nuclear fuel, was undertaken at a site in Oak Ridge, Tennessee, that focused on isolating uranium isotopes.

Although the Manhattan Project had originally been conceived to combat a potential German nuclear weapon, work on the bomb would continue after Germany surrendered on May 8, 1945. U.S. officials were determined to use the bomb against Japan in order to end the war at the earliest possible moment with the fewest casualties.

Secretary of War Henry Stimson told President Harry S. Truman that the bomb could create problems for the United States because it could not maintain a monopoly on the technology. Stimson requested that Truman convene a special committee to consider the implications of the new weapon.

Truman agreed, and the Interim Committee, made up of high-level advisers, held five meetings between May 9 and June 1, 1945. The committee debated the most effective use of the bomb in order to expedite a Japanese surrender.

The committee determined that the weapon should be employed without prior warning, which would increase its psychological impact. The committee suggested that the purpose of the bomb should be to impede the Japanese capacity to wage war and to shock the Japanese with the overwhelming destructive power of the bomb.

Political Implications

The committee also debated the effects of the bomb on postwar international relations. Although the Soviet Union remained aligned with the United States and Great Britain, tensions between the Allies continued to grow, especially over Soviet control of Eastern Europe. The committee fully realized that the bomb could increase the already tense relationship with the Soviet Union.

The committee discussed two ways of handling the issue. The first would be to offer general information to the Soviets about the bomb in order to increase cooperation between the two allies. The other approach would be to use the bomb to gain diplomatic advantages in U.S. dealings with the Soviets, at least for the short term.

The committee was opposed to even providing general information on the bomb to the Soviets and determined that the United States should work to ensure that it stayed ahead of the Soviet Union in the research and production of nuclear weapons.

Truman accepted the committee's findings. For several months Truman had delayed a conference with Joseph Stalin and Winston Churchill until after a successful test of the plutonian bomb, planned for July, believing that a successful test would improve his bargaining position.

On July 16, 1945, the United States successfully exploded the first nuclear bomb in a test code-named Trinity at Alamogordo, New Mexico. The force of the bomb equaled 18,600 tons of TNT, approximately 2,000 times more powerful than the British "Grand Slam," the largest conventional bomb used in World War II.

Ultimatum

15-kiloton uranium bomb nicknamed Little Boy
15-kiloton uranium bomb nicknamed Little Boy

At the end of the conference the Allies presented an ultimatum to Japan in what is known as the Potsdam Declaration. The declaration called on Japan to unconditionally surrender to the Allies or face "prompt and utter destruction." The United States elected not to specifically refer to the atomic bomb by name.

After Japan refused to surrender, Truman made the decision to drop atomic bombs on the Japanese home islands. The Manhattan Project instituted Project Alberta, which involved the wartime delivery of the completed bomb. Research groups were sent to Tinian, an island in the Pacific, which was the base from which the planes carrying the atomic weapons would ultimately depart.

On August 6 at 8:15 a.m., the Enola Gay, piloted by Brigadier General Paul W. Tibbets, released a 15-kiloton uranium bomb nicknamed Little Boy 31,060 feet over the city of Hiroshima, Japan; 43 seconds later the bomb exploded 1,900 feet above the city.

Atomic bombing of Hiroshima
Atomic bombing of Hiroshima

Witnesses reported seeing a searing flash of light, hearing a deafening roar, and feeling a massive rush of air. The 4.4 square miles surrounding the point of detonation were completely destroyed. Estimates suggest that over 60,000 people died immediately, while possibly 70,000 more were to die over the next few years, many from acute exposure to radiation.

Three days later, on August 9, Bock's Car, piloted by Major Charles Sweeny, dropped a 21-kiloton bomb nicknamed Fat Man on Nagasaki. Originally, the mission had been to bomb the Japanese city of Kokura, but the crew was unable to do so because of a heavy haze.

Instead, the plane went to its secondary target. Estimates suggest that 38,000 were killed immediately, with an estimated 35,000 additional fatalities as a result of injuries sustained during the bombing.

In the aftermath of the bombings and the Soviet invasion of the Japanese colony of Manchuria, Emperor Hirohito broke a deadlock in the Supreme Council to accept the Potsdam Declaration as the basis for the Japanese surrender.

The sole Japanese condition was that the emperor be allowed to retain his throne as titular ruler of the people. The Japanese government accepted the terms of surrender on August 15 and formally surrendered to General Douglas MacArthur in Tokyo Bay aboard the battleship USS Missouri on September 2.