While Albert Einstein's letter to President Franklin D. Roosevelt in 1939 played a significant role in bringing attention to the potential development of atomic weapons, it is important to note that the United States was already aware of the scientific possibilities of nuclear fission. Several prominent American physicists, including Enrico Fermi and Leo Szilard, had already recognized the potential military applications of atomic energy.
Einstein's letter did serve as an important catalyst, emphasizing the urgency of atomic research and highlighting the potential of German scientists to develop atomic weapons. It helped to raise awareness among policymakers and led to the establishment of the Manhattan Project, a top-secret research program that aimed to develop atomic bombs. However, it is likely that even without Einstein's direct involvement, the United States would have pursued the development of atomic weapons due to the strategic implications and the broader scientific understanding of the time.
The geopolitical climate of World War II, coupled with the knowledge that Germany was actively researching atomic energy, created a strong impetus for the United States to invest in atomic bomb development. Einstein's involvement certainly added to the momentum, but it is reasonable to assume that the U.S. would have pursued such a project regardless, given the potential military advantage that atomic weapons could provide.