The atomic bomb was ultimately used against Japan, but it was built as a response to a German threat. In late 1938, German scientists discovered how to split ("fission") the uranium atom, releasing nuclear energy. When physicists in the United States learned of this discovery, many feared that Hitler might acquire a frightening new weapon: an atomic bomb. Refugees from the Nazis, most notably the Hungarian physicists Leo Szilard and Eugene Wigner, feared this possibility so much that they began the search for a way to warn Western governments.

Searching for a way to warn the U.S. government, Szilard and Wigner sought the help of the famous physicist Albert Einstein, himself a refugee from Nazi Germany. In August 1939, Einstein signed a letter to President Franklin Delano Roosevelt regarding the possibility of creating an atomic bomb. It was conveyed to Roosevelt in October. The letter helped initiate the American atomic bomb project, but the United States did not immediately begin a crash program to build nuclear weapons. Until 194 1, efforts proceeded quite slowly. Leo Szilard (1895-1964).- In 1933 the Hungarian refugee physicist first conceived of a nuclear chain reaction as a means of liberating atomic energy and creating an atomic bomb. He had only recently left Germany because of Hitler's rise to power. During World War II, Szilard worked for the Manhattan Project's Chicago laboratory. Throughout his life he believed that scientists needed to take a leading political role in society. After the war, he devoted much of his energy to warning the world of the dangers of the nuclear arms race.

In 1941, even before the Japanese attack on Pearl Harbor, the American atomic bomb program was accelerating. Independent research in Britain strongly supported the feasibility of a bomb. Furthermore, Vannevar Bush, the head of American civilian scientific research for the military, received a report that German scientists were pushing ahead on their own bomb project. On October 9, 194 1, President Roosevelt approved intensified research into the feasibility of an atomic bomb.

In June 1942, soon after the American decision to proceed with the atomic bomb, the German authorities unaware of that decision) judged that the huge investment required to produce a bomb was too large for their war economy to support. They also expected to win the war before such an effort would bear fruit. The United States and Britain were unaware of Germany's decision and continued to assume that the Nazis would acquire the atomic bomb, possibly before the Allies did. Japan also investigated nuclear weapons, but its efforts never proceeded beyond small-scale laboratory research and had no impact on the Anglo-American decision to build an atomic bomb. Still, there is little doubt that if Japan (or Germany) had been able to construct such a weapon, it would have been used against the Allies.

In late 1942, Manhattan Project chief Gen. Groves chose physicist J. Robert Oppenheimer to head a new laboratory devoted to the designing atomic bombs. Oppenheimer recommended a remote site in New Mexico for the new facility, where project scientists, many of them world-famous, could work together in complete secrecy. The Los Alamos Laboratory was opened in April 1943. During the last two years of World War II, the Los Alamos staff made a crash effort to design two different kinds of bombs, one using uranium, the other plutonium. The plutonium bomb proved to be Los Alamos' most difficult challenge. [Order of following two paragraphs reversed from original] Leslie R. Groves (I 896-1970) graduated from West Point in 1918 with a degree in civil engineering. During the U.S. military buildup Groves served as the deputy commander of all Army construction projects and was a key figure in the building of the pentagon. On September 17, 1942, he was assigned to lead the Manhattan Project. Though some found him authoritarian, his technical competence and decisive leadership proved essential to the success of the massive program. J Robert Oppenheimer (I 904-1967) was born into a wealthy New York Jewish family and became a brilliant student of theoretical physics. The Nazi persecution of the Jews and the rise of fascism in Europe turned him into an activist with personal ties to Communists--ties that would cost him during the anti-Communist climate of the 1950s. In 1942, he became Groves' inspired choice to head the Los Alamos Laboratory. The young physicist proved to be a superb leader and scientific manager. After the war, he played an important role in advising the U.S. government about nuclear weapons.

The few decision-makers who knew about the Manhattan Project always assumed that the bomb would be used against either Germany or Japan. Some, like Major General Groves, thought that it could be decisive in ending the war. That alone could justify the United States huge investment in the bomb--$2 billion, or roughly $20 billion in 1990s dollars--but the project's great expense also motivated him to have it ready as soon as possible. In the spring of 1945, Groves accelerated the production of fissionable materials. "At no time, from 1941 to 1945 did I ever hear it suggested by the President, or any other responsible member of the government, that atomic energy should not be used in the war." Henry Stimson, Secretary of War (1940-1945)

'If this weapon fizzles, each of you can look forward to a lifetime of testifying before congressional investigating committees." Gen. Groves to his staff, December 24, 1944

The Manhattan Project produced two different types of atomic bombs. The "Little Boy" type, which was dropped on Hiroshima, triggered a nuclear explosion by firing one piece of uranium 235 into another. The "Fat Man" type, which was dropped on Nagasaki, was more complex. It contained a sphere of the metal plutonium 239, around which were arrayed blocks of high explosives. These were designed to produce a highly accurate and symmetrical implosion, which would compress the plutonium sphere to a critical density and set off a nuclear chain-reaction. Scientists at Los Alamos were not entirely confident in the in the plutonium bomb design, so they scheduled a test of "Fat Man" for July 1945.

The atomic nucleus contains elementary particles called protons and neutrons. The nuclear energy holding them together is thousands of times stronger than the chemical energy binding atoms together in molecules (like TNT, for example). For certain very heavy elements (uranium 235 and plutonium 239), the nucleus is almost unstable, When hit by a neutron, it will split, or "fission," into two smaller nuclei, which fly violently apart, releasing nuclear energy and more neutrons. If a "critical mass" of such an element (a few kilograms) is rapidly brought together in a bomb, the average neutron cannot escape from the mass before it hits and splits another nucleus. This releases more neutrons, each of which repeats the process. The resulting runaway nuclear "chain reaction" bums through the fuel in a few millions of a second, liberating energy equal to that in many tons of conventional explosives. All nuclear weapons use fission as the basic process for making a nuclear explosion. Most current nuclear weapons, however, use a fission bomb to trigger the "fusion" of hydrogen nuclei. The resulting "thermonuclear" or "hydrogen bombs" are far more efficient and destructive than atomic bombs. There is in principle no limit to the power of hydrogen bombs.

Natural uranium is mostly uranium 238, mixed with a small amount of uranium 235, the isotope used in bombs. Because the two forms are chemically identical (differing by three neutrons), the problem of purifying enough pure uranium 23 5 to make a bomb was very difficult and required an enormous industrial plant. When enough uranium 235 is brought together, the resulting fission chain reaction can produce a nuclear explosion. But the "critical mass" must be assembled very rapidly; otherwise the heat released at the start of the reaction will blow the fuel apart before most of it is consumed. To prevent such an inefficient "pre-detonation," the uranium bomb uses a gun to fire one piece of uranium 23 5 down the barrel into another. An atomic bomb can also use plutonium. If natural uranium is assembled in a specially constructed pile (or reactor), its own radioactivity converts some of the uranium 238 into a new element called plutonium 239. Plutonium is chemically distinct from uranium and easily separated. But it naturally emits so many neutrons that even the gun-type bomb would be too slow to prevent "pre-detonation." This problem was solved by using a symmetrical shell of explosives to squeeze a plutonium sphere. The implosion instantly increases the plutonium's density, which traps the neutrons inside and causes a runaway chain reaction. For the uranium bomb, the fuel was very difficult to produce but relatively easy to explode. For the plutonium bomb, the situation was the opposite. The Manhattan Project pursued both tracks simultaneously.

"The Most Terrible Weapon Ever Known in Human History"

On April 12, 1945, President Roosevelt died unexpectedly in Warm Springs, Georgia. Vice-President Harry S. Truman, in office for less than three months, was sworn in the same day. Truman was quickly confronted with the need to approve the use of the atomic bomb, which was expected to be ready by August. Truman confronted a complicated situation in Europe and in the Far East. Japan, although weakened, was not willing to surrender. The atomic bomb offered a way to change that. A bloody invasion loomed if atomic bombs did not force the Japanese to surrender.

President Truman came into office with no knowledge of the atomic bomb, because Roosevelt had never told him about it. Shortly after Truman's swearing-in, Secretary of War Henry Stimson mentioned it to him briefly. On April 25, Stimson and Groves gave him a more extensive briefing. Truman had inherited a project that had always aimed at making a practical weapon. He saw the atomic bomb principally as a means to end the war quickly and save American lives. Senator Harry S. Truman (1884-1972): A World War I veteran and Missouri farmer and politician, Truman achieved prominence in the U.S. Senate as chairman of the powerful Truman Committee, which watched over the U.S. industrial and military buildup during World War II As president, he held ultimate responsibility for the decision to use the atomic bomb. Secretary of War Henry L. Stimson (1867-1950): A prominent statesman for over 40 years, Stimson served as Secretary of War for William Howard Taft, Governor-General of the Philippines for Calvin Coolidge, and Secretary of State for Herbert Hoover. Although Stimson was a lifelong Republican, he became Roosevelt's Secretary of War in 1940 and soon became a key policy advisor on the atomic bomb.

On April 5, 1945, one week before Rooseveft's death, Japanese Prime Minister Kuniaki Koiso and his Cabinet resigned because of the increasingly disastrous course of the war--the second such resignation in less than a year. A peace faction in the military-dominated Japanese government had begun to realize that a way had to be found to negotiate an end to the war. The Allied demand for "unconditional surrender" was, however, regarded as intolerable. Emperor Hirohito approved the appointment of the aged Admiral Kantaro Suzuki as the new Prime Minister. But Suzuki's government was hobbled by severe tensions between the peace faction and militarists who vowed to fight to the bitter end. As a result, direct negotiations with the United States could not be undertaken, and Japan lost an opportunity to try to end the war early.

The Soviet Union and Japan had remained at peace, although they were allied with opposite sides in the European war. In the fall of 1944, growing desperation drove the Japanese government to approach Joseph Stalin's communist regime for help in fending off defeat. After the Suzuki cabinet was appointed in April 1945, these initiatives were renewed. Two key civilian politicians -Marquis Kido, the Emperor's closest adviser, and Shigenori Togo, the new Foreign Minister-hoped to use this initiative to negotiate a conditional surrender with the Allies. But they had to conceal this intention from the militarists who vowed to fight on until the Allies gave Japan more concession. . As a result, the Moscow initiative remained weak and indecisive. Emperor Mrohito (1901-1989): A retiring and bookish man, the Emperor had traditionally been portrayed as a "living god" who exercised little real authority over affairs of state. The reality was more complex. While he was opposed to war with the United States and Britain prior to 194 1, he did not discourage Japanese expansionist policies in Asia. Although he tentatively encouraged the Moscow peace initiative in 1945, he also listened to military advisors who argued that one final victory would force Allied leaders to offer improved peace terms. He failed to take decisive action until the atomic bombs had been dropped and the Soviets had declared war.

A key obstacle to any Japanese surrender was the Emperor's position. To the Japanese warlords, the Allied demand for unconditional surrender meant the total destruction of their political system, including a "divine" monarchy that had survived for more than a thousand years. To most Americans, Hirohito was a hated symbol of Japanese military aggression. Many wanted him executed, or at least imprisoned. Undersecretary of State Joseph Grew asserted that the Emperor would be "the sole stabilizing force" capable of making the Japanese armed forces accept a surrender order. Truman ultimately did not accept Grew's advice because he foresaw much resistance to modifying the Allied policy. Joseph G. Grew (1880-1965) was the last U.S. Ambassador to Tokyo before the war. In 1944-45 he served as deputy head of the State Department and Acting Secretary of State. Grew understood the mentality of the Japanese leadership and wanted to end the war early in part to minimize Soviet influence in Asia.

The demand that the Axis powers surrender unconditionally was first proposed by President Roosevelt at the Casablanca Conference in early 1943. This policy was quickly accepted by the Allies because it made war aims clear. It became especially important in the troubled relationship between the Western powers and the Soviet Union. It reassured Soviet dictator Joseph Stalin, who suspected Britain and the United States of wanting to make a compromise peace with the Nazis, leaving his country to bear the brunt of the German war machine. Unconditional surrender was also a popular policy in America, because of the fear that anything less than total victory would fail to root out the causes of fascism and militarism in Germany, Italy, and Japan--just as the Versailles Treaty after World War I had failed to prevent the resurgence of German power.

Truman's decision to drop the atomic bomb was based on saving American lives and shortening the war. However, Joseph Stalin's Union of Soviet Socialist Republics was a factor in American calculations regarding the new weapon and the Japanese. The alliance of the United States, the British Commonwealth, and the Soviet Union, which was forged only after Germany attacked Russia in 1941, was one of convenience. Suspicion between West and East remained high, despite positive feelings evoked by their common struggle against the Nazis. In the spring of 1945, tensions were rising over the Soviet imposition of puppet governments in Eastern Europe after the German defeat. There was also the prospect of similar Soviet gains in the Far East. While U.S. military leaders argued that Soviet entry into the Pacific war must precede the U.S. invasion of Japan, some of Truman's civilian advisers began to question its desirability. Joseph V. Stalin (1879-1953) became the undisputed leader of the Soviet Communist Party and dictator of the Soviet Union during the 1920s. His bloody purges cost millions of lives. He acquired a new international importance as a result of his country's decisive contribution to the defeat of Nazi Germany in World War 11. Stalin authorized a Soviet atomic bomb project in 1942, but did not give it the highest priority until after the United States' atomic bombings of Japanese cities in August 1945.

While the Soviet Union was preoccupied with battling Germany, and Japan was fighting to preserve its conquests in Asia and the Pacific, neither power had an interest in disturbing their mutual peace. But as the defeat of the Nazis approached, the United States wanted the Soviets to attack and pin down the huge Japanese Army in China, which would prevent it from assisting the defense of the Japanese home islands. At the Yalta conference in February 1945, Stalin promised to enter the Pacific war two to three months after Germany's surrender. During the spring of 1945, some American leaders began to doubt the wisdom of this policy. The U.S. Navy's blockade of Japan was nearly complete by April, making troop transfer from China more difficult. Key advisers to President Truman also began to worry about the spread of Communism in post-war Asia. Indeed, Stalin was interested in joining the Pacific war so that he could bring China and Korea into the Soviet sphere and share in the occupation of Japan.

The Manhattan Project was a joint undertaking of the United States, Great Britain and Canada, although dominated by American resources and personnel. President Roosevelt and British Prime Minister Churchill decided to conceal the project from Stalin, hoping to delay Soviet acquisition of nuclear weapons. However, Soviet spies sent atomic secrets back to Moscow. As the time to test and use the bomb approached, the Western Allies had to decide whether to tell Stalin before dropping it on Japan and what post-war nuclear policy should be. Some scientists and advisers, concerned with America's postwar position after the use of the weapon, urged that atomic weapons be placed under "international control" so that a nuclear arms race might be avoided. Others saw advantages in an American or Anglo-American nuclear monopoly.

As tensions grew in spring of 1945 over the Soviet domination of Poland and other Eastern European countries, Secretary of War Stimson hoped that American possession of the atomic bomb power might help make the Soviets "play ball" in Europe and elsewhere. But it was Truman's new Secretary of State, James "Jimmy" Byrnes, who, more than anyone else, recommended a hard line against Stalin's demands for concessions in Europe and Asia. James F. Byrnes (1879-1972) was one of the most powerful figures in the Roosevelt and Truman administrations. As a U.S. senator from South Carolina, he proved instrumental in the passage of Roosevelt's "New Deal. " He became a U. S. Supreme Court justice in 1941, but resigned a year later to lead the war mobilization effort. He was so influential in domestic policy that the press nicknamed him "Assistant President." Truman, cast into the presidency with almost no foreign policy experience, sought out Byrnes as an adviser and as Secretary of State.

While plans for the invasion of Japan were going ahead, preparations were also being made for the military deployment of the atomic bomb. Target recommendations were made by the Target Committee controlled by General Groves and his Manhattan Project staff. Among its primary concerns was showing off the bomb's power to the maximum effect and making the greatest impression possible on the Japanese with the goal of shocking Japan into surrender. To ensure an accurate drop, the Committee insisted that the bombings occur in daylight and clear weather. They also decided that the targets would be a city undamaged by conventional bombing and had geographical layouts that would maximize damage from the bomb's blast wave. By the end of May 1945, the Committee selected, in order of priority, Kyoto, Hiroshima, Kokura and Niigata. The Army Air Forces were ordered not to firebomb these cities.

Kyoto, the top choice of Major General Groves' Target Committee, was never bombed. On May 30, 1945, Groves met Secretary of War Stimson, who asked for the target list. Stimson vetoed Kyoto because it "was he ancient capital of Japan, a historical city, and one that was of great religious significance to the Japanese." He had visited the city several times and was "very much impressed by its ancient culture." Stimson was concerned that the destroying Kyoto would permanently embitter the Japanese against the United States and increase Soviet influence in Japan. Groves argued that Kyoto had a population of over a million, did much war work and had a highly suitable geography for the bomb. He fought for two months to reinstate the city to the target list, but to no avail. In July the port city of Nagasaki was added instead.

The question of whether to drop the first atomic bomb on Japan without warning was left to another group, the Interim Committee on post-war atomic policy. On May 31, 1945, Secretary Stimson chaired a meeting of this group, which included Truman's personal representative, James F. Byrnes, and the committee's scientific advisers, headed by Dr. Robert Oppenheimer. The committee members briefly discussed warning the Japanese to evacuate the target, or arranging a demonstration of the bomb for delegates from Japan. However, they rejected those ideas because they reasoned that the Japanese, if warned, might try to shoot down the bomber or move prisoners of war into the target area, and because the demonstration bomb might fail to explode. Others who know about the atomic bomb were also thinking of ways to demonstrate it. For example, Manhattan Project physicist Edward Teller proposed exploding the first bomb high over Tokyo Bay at night, without any warning, to shock the Japanese leaders. But prior to the first test, the scientists had generally underestimated the power of the bomb, and it was not clear that any non-lethal demonstration would sufficiently impress the Japanese.

Many of the decision-makers knowledgeable about the bomb did not consider it drastically different from conventional strategic bombing, which had already killed hundreds of thousands of civilians throughout the world. Nor was there any guarantee that the bomb would automatically end the war. When Oppenheimer suggested on May 31 that several atomic attacks be carried out on the same day to shock the Japanese, Groves opposed the idea on the grounds that "the effect would not be sufficiently distinct from our regular air force [bombing] program." At that time, the firebombing of Japan had already devastated many cities. The explosive power of the first atomic bombs was also estimated at only 1/10th to 1/2 of what it turned out to be, and no one had a clear impression of the heat and radiation effects.

Leo Szilard and other Manhattan Project scientists there felt that the bomb project had been primarily a response to a threat from Germany. Attacking Japan without first providing a warning and an opportunity to surrender, they felt, would weaken "our moral position ... in the eyes of the world." They were equally concerned that using the bomb without telling the Soviets first would increase the chances of and uncontrolled nuclear arms race after the war. The Chicago group wrote a report, sent petitions to President Truman, and approached Truman's adviser and choice for Secretary of State, James Byrnes. But the President did not receive the petitions before the bomb was used and all the scientists' initiatives were obstructed by Byrnes, Groves, Oppenheimer, and others.

American planning for an invasion of Japan continued in spring of 1945. The Manhattan Project was so secret that most military planners were unaware of it, and the effects of the new weapon on the Japanese were uncertain. Under the leadership of Army Chief of Staff Gen. George C. Marshall, the War Department continued to assume that an invasion would be necessary to force Japan to surrender. Not everyone in the U.S. military agreed. Some Navy officers believed that the blockade could force Japan to quit the war, while many in the Army Air Forces thought firebombing could force surrender by itself or in conjunction with the blockade. Both groups pointed to the terrible casualties of the Okinawa campaign--48,000 American dead and wounded-- in arguing against an invasion. General Marshall and his staff also feared heavy losses but argued that, as with Germany, only the occupation of the enemy's territory and capital would end the war. 

On June 18, 1945, President Truman gave preliminary approval to the invasion plans presented by General Marshall. "Operation Downfall" would have two parts. On or about November 1, 1945, 767,000 Marines and Army troops would begin landing on the beaches of the southern island of Kyushu in "Operation Olympic. " The invasion fleet would be larger than that of the landings in Normandy in June 1944. The objective of this operation would be to occupy the southern half of Kyushu and use it as an air base and staging area for a second invasion. If the Japanese did not then surrender, "Operation Coronet"--the landings on the main island of Honshu--would begin on or about March 1, 1946. A huge force of 28 divisions, twice the size of "Olympic," would eventually come ashore on beaches near Tokyo. Some strategists assumed that it could take until the end of 1946 to occupy the capital and enough of Honshu to force Japan to surrender. Gen. George C. Marshall (1880-1959) played a critical role in expanding the small, poorly armed U.S. Army of 1939 into the massive, effective force of 1942-1945. During the war he was Chief of Staff of the Army, a key strategist in Allied plans on all fronts, and an important adviser to Roosevelt and Truman on the Manhattan Project. After his retirement from the Army, he became Secretary of State in 1947. He won the Nobel Peace Prize in 1953 for the Marshall Plan, which helped to revive the economies of Western Europe.

Estimates of the number of American casualties--dead, wounded, and missing--that the planned invasion of Japan would have cost varied greatly. In a June 18, 1945, meeting, General Marshall told President Truman that the first 30 days of the invasion of Kyushu could result in 31,000 casualties. But Admiral Leahy pointed out that the huge invasion force could sustain losses proportional to those on Okinawa, making the operation much more costly. Had the Kyushu invasion failed to force Japan to surrender, the United States planned to invade the main island of Honshu, with the goal of capturing Tokyo. Losses would have escalated. After the war, Truman often said that the invasion of Japan could have cost half a million or a million American casualties. The origin of these figures is uncertain, but Truman knew that Japan had some two million troops defending the home islands. He believed, along with the many Americans who would have had to invade Japan, that such a campaign might have become, in his words from June 18, 1945, "an Okinawa from one end of Japan to the other." Added to the American losses would have been several times as many Japanese casualties--military and civilian. The Allies and Asian countries occupied by Japan would also have lost many additional lives. For Truman, even the lowest of the casualty estimates was unacceptable. To prevent an invasion and to save as many lives as possible, he chose to use the atomic bomb.

In mid-July 1945, as Manhattan Project scientists prepared for the world's first nuclear explosion, Allied leaders were assembling outside Berlin for the Potsdam Conference. The conference was called to discuss the peace settlement in Europe and to issue a surrender ultimatum to Japan. President Truman had delayed the conference so that it would take place at the time the bomb was to be tested. At Potsdam he gave final verbal approval for dropping the atomic bomb if Japan rejected the ultimatum.

At 5:29:45 a.m., July 16, 1945, a blinding flash and unbelievable heat seared the New Mexico desert--the world's first nuclear explosion. Codenamed "Trinity," the Manhattan Project's test of the plutonium implosion bomb was a stunning success. The explosion almost equaled 20,000 tons of TNT, many times what some had expected. General Groves and his Project leaders were jubilant and relieved. But for some, the spectacle also cast an ominous shadow. Los Alamos scientific director Dr. Robert Oppenheimer later said he thought of the lines from the Hindu scripture, the Bhagavad Gita, "I am become Death, Destroyer of Worlds."

On July 26, 1945, the three largest Allied powers at war in the Pacific, the United States, Britain, and China, issued the Potsdam Declaration, which demanded that the Japanese Empire surrender immediately or face "prompt and utter destruction." Because of potential Allied and domestic opposition to anything less than "unconditional surrender," the declaration contained no reference to retaining Emperor Hirohito on the throne. Nor, for reasons of military secrecy, did it contain any direct reference to the atomic bomb or Soviet entry into the war. The declaration did not change the position of the Japanese government. The military's reaction was especially unfavorable. On July 28, Prime Minister Suzuki announced that his government would ignore ("mokatsu") the Declaration. As a result, the United States used the atomic bomb.