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 wrote to US President Franklin D Roosevelt in 1939 to urge him to try and develop the atomic bomb.
• Einstein and other leading scientists were concerned that Nazi Germany could use nuclear energy to build an "extremely powerful bombs of a new type" that could destroy entire ports.
• His letter helped pave the way for the Manhattan Project, which developed the atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki killing an estimated 200,000 people and effectively ending WWII.
• Einstein later said he had felt no choice but to encourage the US to develop the technology, but he called the letter his "one great mistake in my life" having learned Germany was never close to developing atomic bombs.
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On August 2 1939, one month before the outbreak of World War II, Albert Einstein, the famous German-born physicist, signed a two-page letter to US President Franklin D Roosevelt that would bring the US into the nuclear arms race and change the course of history.

Einstein, possibly the most famous scientist in history, was already in the US having fled Germany when the Nazis came to power, and learned that German scientists had discovered nuclear fission - splitting a large atomic nucleus to release energy.

He signed a letter to Roosevelt, warning 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.

Read more: 'The most intimidating minute I have ever spent': The first nuclear weapon test was 74 years ago - here's what it was like to watch the 'shocking' explosion

He urged Roosevelt to speed up uranium research in the US in what would become known as "The God Letter" and that Einstein would later call his "one great mistake."

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

Einstein's warnings were read to Roosevelt by a man called Alexander Sachs, who also read out other warnings about such a bomb to the president.

Roosevelt responded by saying: "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: "This requires action."

Einstein, who was Jewish, had been encouraged to write to Roosevelt by Hungarian physicist Leo Szilard, 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.

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

Read more: 15 Albert Einstein quotes that reveal the mind of a true genius

And Einstein himself never gave any details about how that energy could be harnessed, and 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 an effect, with Roosevelt creating the Advisory Committee on Uranium in October 1939, the same month he received Einstein's letter. By that point, World War II had already broken out, although 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.

Read more: Hiroshima: The moment the US deployed the most powerful weapon known to man

Japan informally surrendered to the Allied forces eight days later, effectively ending World War II.

Nazi Germany never succeeded in making nuclear weapons - and it seemed they 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 to big a security risk as he was both German and had a history as a left-leaning political activist.

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

Einstein later told American chemist and activist Linus Pauling: "Had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing for the bomb."

After the bombings in Japan, he warned that "we thus drift toward unparalleled catastrophe."

Letters uncovered in 2005 showed 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: "I was well aware of the dreadful danger for all mankind, if these experiments would succeed."

"I did not see any other way out."

Read more: Rare footage shows the successful testing of the most powerful weapon known to man

So crucial was Einstein's letter that investing legend Warren Buffett told students at Columbia University in 2017: "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 to 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 [illegible] 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 unofficial capacity. His task might comprise the following:

a) To approach Government Departments, keep them informed of the further development, and out forward recommendations for Government action, giving particular attention to the problem 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 a contribution 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 Weishlicker [sic], is attached to the Kaiser Wilhelm Institute in Berlin where some of the American work on uranium is now being repeated.

Yours very truly,

Albert Einstein