A letter from Albert Einstein to President Franklin Roosevelt, warning about Nazi Germany's nuclear ambitions, has sold at auction for $3.992 million. The letter, dated August 2, 1939, urged Roosevelt to accelerate U.S. nuclear research, which ultimately contributed to the Manhattan Project. This version of the letter was previously owned by Microsoft co-founder Paul Allen, who acquired it for $2.1 million in 2002. The original letter is preserved in the Franklin D. Roosevelt Library in New York, while the auctioned version is the only one in private hands. Christie's auction house highlighted the letter as one of the most influential letters in history, reflecting Einstein's concerns about the potential for devastating nuclear weapons. Despite its significant sale price, it does not surpass the record for Einstein memorabilia, which stands at $13 million for another manuscript.