August 2015 marks the 70th anniversary of the atomic bombings of Hiroshima and Nagasaki. These bombs, the products of the United States Army's Manhattan Project, helped to end World War II and had enormous long-term effects on global political strategies by setting the stage for the Cold War and nuclear proliferation. This article explores the context and legacy of the Manhattan Project. The state of the war in the summer of 1945 is described, as are how the target cities came to be chosen, deliberations surrounding whether the bombs should be used directly or demonstrated first, and the long-term effects of the Project on individual scientists, the relationship between scientists and society, the subsequent development of nuclear arsenals around the world, and the current status of these arsenals and how they might evolve in the future.