The Alsos Mission was an organized effort by a team of British and United States military, scientific, and intelligence personnel to discover enemy scientific developments during World War II. Its chief focus to investigate the progress that Nazi Germany was making in the area of nuclear technology, and to seize any German nuclear resources that would either be of use to the Manhattan Project or worth denying to the Soviet Union. It also investigated German chemical and biological weapon development and the means to deliver them, and any other advanced Axis technology it was able to get information about in the course of the other investigations.

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.