Albert Einstein wrote to the US pleading with the government to build an atomic bomb 80 years ago. Here's what he said.

• Albert Einstein was famously a pacifist, but he urged the US to develop the atomic bomb.
• This helped pave the way for the Manhattan Project, which developed the bombs dropped on Japan.
• He worried Nazi Germany was developing nuclear weapons, but later learned they were far behind the US.

On August 2, 1939, one month before the outbreak of World War II, Albert Einstein signed a two-page letter to US President Franklin D. Roosevelt that would help bring the US into the nuclear arms race and change the course of history.

Einstein, the famous German-born physicist, was already in the US, having fled Germany when the Nazis came to power. He learned that German scientists had discovered nuclear fission, the process of splitting an atom's nucleus to release energy.

The letter warned Roosevelt that "extremely powerful bombs of a new type" could be created in light of this discovery — and that these bombs would be capable of destroying entire ports and their surrounding areas.

The letter — which Einstein would later call his "one great mistake" — urged Roosevelt to speed up uranium research in the US.

You can read it here, or read a full transcript at the bottom of this article:

Einstein's warnings were read to Roosevelt by a man named Alexander Sachs, who also read out other warnings about such a bomb to the president, The New York Times reported at the time.

Roosevelt said, "Alex, what you are after is to see that the Nazis don't blow us up."

Sachs responded with a single word: "Precisely."

Roosevelt then called in his secretary and told him that "this requires action."

Einstein, who was Jewish, had been encouraged to write to Roosevelt by Leo Szilard, the Hungarian-born physicist who was convinced that Germany could use this newly discovered technology to create weapons.

Szilard and two other Hungarian physicists, Edward Teller and Eugene Wigner, who were both refugees, told Einstein of their grave concerns.

Szilard wrote the letter, but Einstein signed it, as they believed he had the most authority with the president.

Cynthia Kelly, the president of the Atomic Heritage Foundation, told National Geographic in 2017 that while Einstein's famous discovery that energy and mass were different forms of the same thing had set the stage for this kind of creation, "he certainly was not thinking about this theory as a weapon."

And Einstein never gave any details about how that energy could be harnessed, once saying: "I do not consider myself the father of the release of atomic energy. My part in it was quite indirect."

Einstein's letter had a notable impact: Roosevelt created the Advisory Committee on Uranium in October 1939, the same month he received Einstein's letter.

By that point, World War II had broken out, though the US was not yet involved.

The committee later morphed into the Manhattan Project, the secret US committee that developed the atomic bombs that were dropped on the Japanese cities of Hiroshima and Nagasaki in 1945, killing an estimated 200,000 people.

Days after the bombings, Japan informally surrendered to the Allied forces, effectively ending World War II.

Nazi Germany never succeeded in making nuclear weapons — and it seemed it never really tried.

Einstein was not involved in the bomb's creation. He was not allowed to work on the Manhattan Project — he was deemed too big a security risk, as he was both German and had been known as a left-leaning political activist.

But when he heard that the bomb had been used in Japan, he said, "Woe is me."

Einstein later said, "Had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing for the bomb."

He also warned that "we thus drift toward unparalleled catastrophe."

In letter published in 2005, he wrote to a Japanese friend: "I have always condemned the use of the atomic bomb against Japan but I could not do anything at all to prevent that fateful decision."

And he wrote in a Japanese magazine in 1952 that he was "well aware of the dreadful danger for all mankind, if these experiments would succeed."

"I did not see any other way out," he wrote.

So crucial was Einstein's letter that the investing legend Warren Buffett told students at Columbia University in 2017 that "if you think about it, we are sitting here, in part, because of two Jewish immigrants who in 1939 in August signed the most important letter perhaps in the history of the United States."

Here's a full transcript of what Einstein sent Roosevelt:

Sir:

Some recent work by E. Fermi and L. Szilard, 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 Szilard 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 moderate quantities. There is some good ore in Canada and the former Czechoslovakia, while the most important source of uranium is Belgian Congo.

In view of this situation you may think it desirable to have some 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 inofficial 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