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 () or (). The first effort started in April 1939, just months after the discovery of nuclear fission in Berlin in December 1938, but ended shortly ahead of the September 1939 German invasion of Poland, for which many German physicists were drafted into the . A second effort under the administrative purview of the 's began on September 1, 1939, the day of the invasion of Poland. The program eventually expanded into three main efforts: (nuclear reactor) 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 turned the program over to the Reich Research Council () while continuing to fund the activity.

The program was split up among nine major institutes where the directors dominated research and set their own objectives. Subsequently, the number of scientists working on applied nuclear fission began to diminish as many researchers applied their talents to more pressing wartime demands. The most influential people in the included Kurt Diebner, Abraham Esau, Walther Gerlach, and Erich Schumann. Schumann was one of the most powerful and influential physicists in Germany. Diebner, throughout the life of the nuclear weapon project, had more control over nuclear fission research than did Walther Bothe, Klaus Clusius, Otto Hahn, Paul Harteck, or Werner Heisenberg. Esau was appointed as Hermann Göring's plenipotentiary for nuclear physics research in December 1942, and was succeeded by Walther Gerlach after he resigned in December 1943.

Politicization of German academia under the Nazi regime of 1933–1945 had driven many physicists, engineers, and mathematicians out of Germany as early as 1933. Those of Jewish heritage who did not leave were quickly purged, further thinning the ranks of researchers. The politicization of the universities, along with Wehrmacht demands for more manpower (many scientists and technical personnel were conscripted, despite possessing technical and engineering skills), substantially reduced the number of able German physicists.

Developments took place in several phases, but in the words of historian Mark Walker, it ultimately became "frozen at the laboratory level" with the "modest goal" to "build a nuclear reactor which could sustain a nuclear fission chain reaction for a significant amount of time and to achieve the complete separation of at least tiny amounts of the uranium isotopes". The scholarly consensus is that it failed to achieve these goals, and that despite fears at the time, the Germans had never been close to producing nuclear weapons. The final attempt at constructing a reactor, the 1945 Haigerloch research reactor, did not achieve criticality. With the war in Europe ending in early 1945, various Allied powers competed with each other to obtain surviving components of the German nuclear industry (personnel, facilities, and materiel), as they did with the pioneering V-2 short-range ballistic missile program. The Western Allied effort was the Alsos Mission and the Soviet effort was Russian Alsos. Due to the success of the Manhattan Project, the Western Allies prioritized denying German nuclear scientists and resources to the nascent Soviet atomic bomb project.

Discovery of nuclear fission

In December 1938, German chemist Otto Hahn and his assistant Fritz Strassmann sent a manuscript to the German science journal Naturwissenschaften ("Natural Sciences") reporting that they had detected and identified the element barium after bombarding uranium with neutrons.O. Hahn and F. Strassmann Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle (On the detection and characteristics of the alkaline earth metals formed by irradiation of uranium with neutrons), Naturwissenschaften Volume 27, Number 1, 11–15 (1939). The authors were identified as associated with the Kaiser-Wilhelm-Institut für Chemie, Berlin-Dahlem. Received 22 December 1938. Their article was published on 6 January 1939. On 19 December 1938, eighteen days before the publication, Otto Hahn communicated these results and his conclusion of a bursting of the uranium nucleus in a letter to his colleague and friend Lise Meitner, who had fled Germany in July to the Netherlands and then to Sweden.Ruth Lewin Sime Lise Meitner's Escape from Germany, American Journal of Physics Volume 58, Number 3, 263–267 (1990). Meitner and her nephew Otto Robert Frisch confirmed Hahn's conclusion of a bursting and correctly interpreted the results as "nuclear fission" – a term coined by Frisch.Lise Meitner and O. R. Frisch Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction, Nature, Volume 143, Number 3615, 239–240 (11 February 1939). The paper is dated 16 January 1939. Meitner is identified as being at the Physical Institute, Academy of Sciences, Stockholm. Frisch is identified as being at the Institute of Theoretical Physics, University of Copenhagen. Frisch confirmed this experimentally on 13 January 1939.O. R. Frisch Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment, Nature, Volume 143, Number 3616, 276–276 (18 February 1939) . The paper is dated 17 January 1939. [The experiment for this letter to the editor was conducted on 13 January 1939; see Richard Rhodes, The Making of the Atomic Bomb 263 and 268 (Simon and Schuster, 1986).]In 1944 Hahn received the Nobel Prize for Chemistry for the discovery and the radiochemical proof of nuclear fission. Some American historians have documented their view of the history of the discovery of nuclear fission and believe Meitner should have been awarded the Nobel Prize with Hahn. See , and

First Uranverein and other early 1939 efforts On 22 April 1939, after hearing a colloquium paper by his colleague Wilhelm Hanle at the University of Göttingen proposing the use of uranium fission in an Uranmaschine (uranium machine, i.e., nuclear reactor), Georg Joos, along with Hanle, notified Wilhelm Dames, at the Reichserziehungsministerium (REM, Reich Ministry of Education), of potential military and economic applications of nuclear energy. Abraham Esau, a physicist at the Reich Research Council of the REM, organized a meeting for what become informally known as a Uranverein (Uranium Club). The group included the physicists Walther Bothe, Robert Döpel, Hans Geiger, Wolfgang Gentner (probably sent by Walther Bothe), Wilhelm Hanle, Gerhard Hoffmann, and Georg Joos; Peter Debye was invited, but he did not attend. After this, informal work began at the Georg-August University of Göttingen by Joos, Hanle, and their colleague Reinhold Mannkopff. Formally the group of physicists was known as the Arbeitsgemeinschaft für Kernphysik (Nuclear Physics Association). This initial work at Göttingen lasted until the fall of 1939, when Joos and Hanle were drafted into other military research...

Independently of this effort, Paul Harteck, director of the physical chemistry department at the University of Hamburg and an advisor to the Heereswaffenamt (HWA, Army Ordnance Office), and his teaching assistant Wilhelm Groth wrote a letter on 24 April 1939 to Army Ordnance which also mentioned the military application of nuclear chain reactions. Harteck would not receive a reply until August 1939, however, as part of the second Uranverein.

Also independently of the first Uranverein, Nikolaus Riehl, the head of the scientific headquarters at Auergesellschaft (usually known as just "Auer"), a German industrial firm, read a June 1939 paper by Siegfried Flügge, on the technical use of nuclear energy from uranium.Siegfried Flügge Kann der Energieinhalt der Atomkerne technisch nutzbar gemacht werden?, Die Naturwissenschaften Volume 27, Issues 23/24, 402–10 (9 June 1939).Also see: Siegfried Flügge Die Ausnutzung der Atomenergie. Vom Laboratoriumsversuch zur Uranmaschine – Forschungsergebnisse in Dahlem, Deutsche Allgemeine Zeitung No. 387, Supplement (15 August 1939). English translation: Document No. 74 Siegfried Flügge: Exploiting Atomic Energy. From the Laboratory Experiment to the Uranium Machine – Research Results in Dahlem [15 August 1939] in . [This article is Flügge's popularized version of the June 1939 article in Die Naturwissenschaften.] As Auer had a substantial amount of uranium on hand as a waste product from the process of extracting radium, Riehl recognized the possibility of uranium production as a business opportunity for the company, and July 1939 contacted the Army Ordnance Office on the matter. Army Ordnance eventually provided an order for the production of uranium oxide, which took place in the Auer plant in Oranienburg, north of Berlin.

These three efforts, as noted, were independent and lasted until the fall of 1939, when the outbreak of World War II disrupted the work at Göttingen, and also prompted the HWA (Army Ordnance) to take over the work from the Reich Research Council. After the fact, this early work was designated as the first Uranverein, with the HWA's reorganized project being designated the second Uranverein.

Second Uranverein

In August 1939, just before the German invasion of Poland precipitated the formal start of World War II, the Army Ordnance Office (HWA) moved to take over the work of the Reichsforschungsrat (RFR, Reich Research Council) of the Reich Education Ministry (REM), and ordered the RFR to halt all experiments and work on nuclear energy. Esau protested that the discovery of nuclear fission was too recent to warrant such an action, but was ignored. These actions were initiated by the physicist Kurt Diebner, an advisor to the HWA, in association with Erich Bagge. In September 1939, Diebner organized a meeting in Berlin on 16 September.

The invitees to this meeting included Walther Bothe, Siegfried Flügge, Hans Geiger, Otto Hahn, Paul Harteck, Gerhard Hoffmann, Josef Mattauch, and Georg Stetter. Its purpose, as Bagge later recalled, was

This group, like the one before it, referred to itself informally as a Uranverein. A second meeting was held soon thereafter, and included Klaus Clusius, Robert Döpel, Werner Heisenberg, and Carl Friedrich von Weizsäcker. Also at this time, the Kaiser-Wilhelm Institut für Physik (KWIP, Kaiser Wilhelm Institute for Physics, after World War II the Max Planck Institute for Physics), in Berlin-Dahlem, was placed under HWA authority, with Diebner as the administrative director, and the military control of the nuclear research commenced. Although the official slogan of the government was "We must make use of physics for warfare", Heisenberg and his colleagues decided that they should instead "make use of warfare for physics".: "But this [a race for the atomic bomb] was not the situation. The situation psychologically was different. It was a new situation for us scientists in Germany. Now for the first time we could get money from our government to do something interesting and we intended to use this situation. The official slogan of the government was: We must make use of physics for warfare. We turned it around for our slogan: We must make use of warfare for physics. It was a very natural reaction. Everybody likes to make interesting experiments."