During World War II, Japan had several programs exploring the use of nuclear fission for military technology, including nuclear reactors and nuclear weapons. Like the similar wartime programs in Nazi Germany, it was relatively small, suffered from an array of problems brought on by lack of resources and wartime disarray, and was ultimately unable to progress beyond the laboratory stage during the war.

Nazi Germany undertook several research programs relating to nuclear technology, including nuclear weapons and nuclear reactors, before and during World War II. These were variously called Uranverein or Uranprojekt. The first effort started in April 1939, just months after the discovery of nuclear fission in Berlin in December 1938, but ended only a few months later, shortly ahead of the September 1939 German invasion of Poland, for which many notable German physicists were drafted into the Wehrmacht. A second effort under the administrative purview of the Wehrmacht's Heereswaffenamt began on September 1, 1939, the day of the invasion of Poland. The program eventually expanded into three main efforts: Uranmaschine development, uranium and heavy water production, and uranium isotope separation. Eventually, the German military determined that nuclear fission would not contribute significantly to the war, and in January 1942 the Heereswaffenamt turned the program over to the Reich Research Council while continuing to fund the activity.