Einstein’s atomic bomb letter, which he called his ‘one great mistake,’ just sold for almost $4 million

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.