In late 1938, two German scientists, Otto Hahn and Fritz Strassmann, discovered a fundamentally new and unexpected phenomenon. After irradiating uranium with neutrons, they were surprised to detect barium, an element only about half of the mass of uranium. The conclusion was inescapable; they had split the uranium atom into two parts. Otto Hahn and Fritz Strassmann had discovered nuclear fission, the splitting of the atom.
    Two physical properties of nuclear fission are particularly interesting - splitting the uranium nucleus with a neutron releases energy and a few additional neutrons. After the discovery of fission, Siegfried Flügge from Germany and Leó Szilárd a Hungarian physicist, independently, realized that those two properties meant the possibility of a nuclear chain reaction. A nuclear chain reaction, if it could be made to occur very quickly, meant that the building of a new weapon of war, the atom bomb, was possible. A slow, controlled chain reaction might be used to generate electric power or to produce fissionable material for atom bombs. As the clouds of World War II darkened over Europe and the rest of the world, scientists focused their attention on learning what was needed to design and manufacture atom bombs.
    In 1905, Albert Einstein, as part of his Special Theory of Relativity, arrived at the conclusion that a large amount of energy could be released from a small amount of matter. This was expressed by the famous equation E = mc2 (energy = mass times the speed of light squared). The atomic bomb would clearly illustrate this principle.
    The release of energy in a nuclear fission is so great that with a relatively small amount of uranium, the energy released would be thousands of times greater than a TNT mass of the same size. But the study of nuclei, in those years the fastest growing area of physics, had little effect on Einstein. In those days, nuclear physicists were gathering into large teams of scientists and technicians, heavily funded by governments and foundations, engaged in experiments to fully understand the secrets of nuclear fission. Such work was alien to Einstein's abstract thought, done with a mathematical formulation. Einstein was not a nuclear physicist as some people might think. In return, experimental nuclear physicists in the 1930s had little need for Einstein's theories.
    In 1933, Einstein, already a prominent German scientist, was accused of treason by the Third Reich, and his books were burned. Albert Einstein, knowing that his chances of continuing physics research in Germany was doomed, sought refuge in the United States.
    In the summer of 1939, Dr. Szilárd who had fled Germany for America, drafted a letter with Albert Einstein to send to President Franklin Roosevelt, under Einstein's signature, to warn Roosevelt that weapons could be created using a nuclear chain reaction in uranium and that it was very likely that Germany had started working on a uranium bomb. This letter led to the formation of the Advisory Committee on Uranium. The committee did little, however, until Rudolf Peierls and Frisch, working in England, made detailed calculations about the feasibility of nuclear weapons and proposed some possible approaches to making an atomic bomb.
    But bombs were not what Einstein had in mind when he published his work on the relationship between matter and energy. Indeed, he considered himself to be a pacifist. In 1929, he publicly declared that if a war broke out, he would "unconditionally refuse to do war service, direct or indirect, regardless of how the cause of the war should be judged." Einstein’s position would change in 1933, as the result of Adolf Hitler's rise to power in Germany. While still promoting peace, Einstein no longer fit his previous self-description of being an "absolute pacifist.”
    Einstein's greatest role in the invention of the atomic bomb was signing the letter to President Franklin Roosevelt urging that the atom bomb be built. The splitting of the uranium atom in Germany, in December 1938, plus continued German aggression, led some physicists to fear that Germany might be working on an atomic bomb. It was also especially troubling that Germany had stopped the sale of uranium ore from occupied Czechoslovakia.
    Among those concerned were physicists like Leó Szilárd and Eugene Wigner. But Szilárd and Wigner had no influence with those in power. So in July 1939 they explained the problem to someone who did - Albert Einstein. According to Szilárd, Einstein said the possibility of a chain reaction, "...never occurred to me,” although Einstein was quick to understand the concept.
    After consulting with Einstein, in August 1939, Szilárd wrote the letter to President Roosevelt with Einstein's signature on it. The letter was delivered to Roosevelt in October 1939 by Alexander Sachs, a friend of the President.
    Germany had invaded Poland the previous month. The time was ripe for action. That October the Briggs Committee was appointed to study uranium chain reactions. Einstein played no other role in the nuclear bomb project. As a German who had supported left-wing causes, he was denied security clearance for such sensitive work when the Manhattan Project to build the Atom Bomb began in 1942. But during the war he did perform useful service as a consultant for the United States Navy's Bureau of Ordnance.
    The atomic bombings of Japan occurred three months after the surrender of Germany. Hiroshima was bombed on 6 August 1945 and Nagasaki on 9 August. The total human cost for these two bombings, were in excess of 250,000 lives.
    Einstein withheld public comment on the atomic bombing of Japan until a year afterward. A short article on the front page of the New York Times contained his view: "Prof. Albert Einstein... said that he was sure that President Roosevelt would have forbidden the atomic bombing of Hiroshima had he been alive and that it was probably carried out to end the Pacific war before Russia could participate."
    In November 1954, five months before his death, Einstein summarized his feelings about his role in the creation of the atomic bomb: "I made one great mistake in my life when I signed the letter to President Roosevelt recommending that atom bombs be made; but there was some justification - the danger that the Germans would make them."
    A copy of Einstein’s letter is shown here:
Albert Einstein
Old Grove Rd. Nassau Point
Peconic, Long Island

August 2nd 1939

F.D. Roosevelt
President of the United States
White House
Washington, D.C.

Sir:
    Some recent work by E. Fermi and L. Szilárd, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations:
    In the course of the last four months it has been made probable - through the work of Joliot in France as well as Fermi and Szilárd in America - that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.
    This new phenomenon would also lead to the construction of bombs, and it is conceivable - though much less certain – that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air.
    The United States has only very poor ores of uranium in moderatequantities. There is some good ore in Canada and the former Czechoslovakia, while the most important source of uranium is Belgian Congo.
    In view of the situation you may think it desirable to have more permanent contact maintained between the Administration and the group of physicists  working on chain reactions in America. One possible way of achieving this might be for you to entrust with this task a person who has your confidence and who could perhaps serve in an un-official capacity. His task might comprise the following:
    a) to approach Government Departments, keep them informed of the further development, and put forward recommendations for Government action, giving particular attention to the problem of securing a supply of uranium ore for the United States;
    b) to speed up the experimental work, which is at present being carried on within the limits of the budgets of University laboratories, by providing funds, if such funds be required, through his contacts with private persons who are willing to make contributions for this cause, and perhaps also by obtaining the co-operation of industrial laboratories which have the necessary equipment.
    I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated.

Yours very truly, (Albert Einstein)