Answer: Albert Einstein
Albert Einstein alerted the then president of the United States, Franklin Delano Roosevelt, of the threat posed by Germany with its possible development of a weapon with nuclear energy.
This letter was written by Leó Szilárd, (who helped in the writing in English) and signed by Einstein to be sent to Roosevelt on August 2nd, 1939.
The letter warned that the results of studies on uranium had shown that it could become a new source of high power (at that time it was already studying the use of uranium in nuclear fission), with which it could even build highly destructive bombs in the immediate future, due to the velocity with which the investigations progressed.
In addition, he warned about the threat posed by Germany to the United States, after learning that one of the largest sources of uranium, located in the Belgian Congo was held by Germany that had recently invaded this area. Therefore, in the letter he urged the president to form a physicists team to work quickly on the development of a bomb with this element.
President Rosevelt took this letter very seriously, after which he created the famous Manhattan Project and the first atomic bomb.
However, Einstein was not allowed to participate in the project because he was considered a threat to the security of the project, due to his pacifist vision (his intention with the letter was only to prevent Germany from developing the bomb first), therefore he was not involved in the project, nor was he aware of the development of the atomic bomb, nor did he influence the decision to launch it.
It is worth noting that Einstein regretted having signed and sent that letter due to the serious consequences brought about by the use of the bomb for war purposes (in Hiroshima on August 6th, 1945) and expressed that if he had known that Germany would fail in its attempts to create this nuclear weapon, he would not have sent the letter.