Einstein's 1939 letter, warning of atomic weapons, just sold at auction for $3.9 million

• Einstein's 1939 letter to Roosevelt, warning of atomic weapons, sold at auction for $3.9 million.
• Einstein later regretted the letter for its ultimate role in helping to develop the atomic bomb.

In the summer of 1939, Albert Einstein made what he considered his "one great mistake": signing a letter that would reach the desk of President Franklin Roosevelt and help usher in the nuclear age.

Now, a copy of that letter, also signed by Einstein, has sold at a Christie's auction for $3.9 million. The original that was sent to Roosevelt is part of the Franklin D. Roosevelt Library's collection in New York.

"This has been described as one of the most influential letters in history," Peter Klarnet, senior specialist for Americana, books, and manuscripts at Christie's, said in a statement.

The recent sale doesn't set a record price for Einstein memorabilia, though. Christie's previously auctioned one of the famed scientist's manuscripts for $13 million.

The letter that launched our nuclear age

The letter warned that Germany could develop "extremely powerful bombs" using uranium and urged the president to "speed up" the US's own work on the element, eventually leading to the Manhattan Project and the development of the world's first atomic bomb.

Though Einstein didn't write the letter, his worldwide esteem meant his signature added significance and credibility to the letter.

The copy that sold at auction on Tuesday was the only version in private hands, according to Christie's. It was part of a larger collection belonging to the late Microsoft co-founder Paul Allen.

Allen bought the letter in 2002 for $2.1 million. Before that, it had belonged to publisher Malcolm Forbes, who acquired it from Hungarian physicist Leo Szilard's estate, The Guardian reported. Szilard wrote the letter, which Einstein signed.

Christie's didn't disclose the identity of the letter's new owner.

Einstein's influence on the Manhattan Project

When Szilard wrote the letter, German scientists had recently discovered nuclear fission, the process of splitting atoms and releasing energy, which could be used to power an atomic bomb.

For Szilard and Einstein, the possibility of the Germans creating a nuclear weapon was reason enough to write President Roosevelt.

Both Szilard and Einstein were Jewish scientists who had fled Europe during Adolf Hitler's rise to power.

Once Germany surrendered, however, Szilard and Einstein no longer felt the use of nuclear weapons was justified.

"Woe is me," Einstein said when he learned of the attacks on Hiroshima and Nagasaki, Japan, in 1945 that killed an estimated 200,000 people.

Though Einstein was never a part of the Manhattan Project — the US Army Intelligence Office denied him the necessary security clearance — the Nobel Prize winner regretted his role anyway.

After the war, he spoke out against nuclear proliferation. Before his death in 1955, Einstein signed a manifesto written by philosopher Bertrand Russell.

Now known as the Russell-Einstein Manifesto, the document warned the public about an even greater threat than the atomic bomb: newly developed hydrogen bombs, which are far more destructive.

The manifesto led to the creation of the Pugwash Conferences on Science and World Affairs, an organization that — to this day — is dedicated to ridding the world of weapons of mass destruction.