A historic letter from Albert Einstein, urging President Franklin D. Roosevelt to develop atomic bombs due to fears of Nazi Germany's nuclear capabilities, is set to be auctioned at Christie's with an estimated value of $4 million. Co-written by physicist Leo Szilard in 1939, the letter was pivotal in prompting the U.S. to initiate the Manhattan Project, leading to the development of atomic bombs used in World War II. The original letter is housed in the Roosevelt Library, while a shorter signed copy, previously owned by Microsoft co-founder Paul Allen, is being auctioned. Despite the letter's historical significance, Einstein later expressed regret over its role in the creation of nuclear weapons.