The scientist warned the US president that Germany might be able to develop an atomic weapon

A letter from Albert Einstein warning the US president that Nazi Germany might be able to develop a nuclear bomb has been sold for nearly $4 million, according to Christie’s auction house.

The warning by the scientist is believed to have resulted in the US launching the Manhattan Project to research and develop nuclear weapons, which led to the first use of atomic bombs ever against Japan in 1945.

Addressed to then-US President Franklin D. Roosevelt (FDR), the two-page document was written and signed by the physicist in 1939, weeks before World War II began. Einstein wrote about Nazi Germany’s work on the nuclear program, suggesting that it could lead to the construction of “extremely powerful bombs.” He called for “quick action,” urging Washington to stockpile uranium ore and begin work on its own atomic weapons.

Einstein’s letter was later expanded by his student, Leo Szilard, and a group of fellow scientists. It was delivered to the White House by hand, and is now part of the permanent collection of the FDR Presidential Library & Museum in New York.

The short letter sold at Christie’s on Tuesday is the original which was saved by Szilard and ended up in the hands of collectors. It was put up for sale as part of an auction of artifacts belonging to Microsoft co-founder Paul Allen, who died in 2018 at the age of 65.

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Einstein reportedly regretted the letter due to its role in making the US the only country (at the time) to produce nuclear weapons. He was quoted in 1947 as saying: “Had I known that the Germans would not succeed in producing an atomic bomb, I would never have lifted a finger.”