The secret project to develop nuclear weapons in the United States during WWII was known as the Manhattan Project. Initiated in response to fears that Nazi Germany might construct an atomic bomb, this project involved a series of research and development efforts led by the U.S. with collaboration from the United Kingdom and Canada. Key figures like J. Robert Oppenheimer and Enrico Fermi were integral to its success. The culmination of the project resulted in the first successful atomic bomb tests in 1945, significantly altering the course of warfare and international relations post-WWII.