Berlin, Germany April 25, 1945

In the same month World War II began in Europe, Septem­ber 1939, the German Army Wea­pons Agency (Heeres­waffen­amt, or HWA) placed all pro­grams asso­ci­ated with the nation’s nas­cent nuclear energy pro­ject under its autho­rity. The wea­pons pro­gram even­tu­ally expanded into three main efforts: setting up a nuclear reac­tor, pro­ducing ura­ni­um and heavy water (deu­terium oxide) (the Allies mounted a bombing and sabo­tage cam­paign against the Norsk Hydro heavy water plant in Ger­man-occu­pied Nor­way), and sepa­ra­ting ura­nium iso­topes. Like­wise, the pres­ti­gious and inde­pen­dent Kaiser Wil­helm Insti­tute for Physics (KWIP) in the Berlin sub­urb of Dah­lem was placed under HWA autho­rity. (German-born the­o­ret­i­cal phys­i­cist Albert Ein­stein served as KWIP’s head from 1914 to 1932.) In June 1942 the pro­gram, along with its seventy scien­tists, was handed off to the Reich Minis­try for Armament and Ammunition under Albert Speer.

Toward the end of the war, the Allied powers com­peted to ob­tain sur­viving com­po­nents of the Nazis’ nuclear wea­pons pro­gram, just as they famously did with the V‑2 bal­listic rocket pro­gram. The Soviets used special search teams in con­quered ter­ri­to­ries to iden­tify and “requi­si­tion” equip­ment, mate­riel, intel­lec­tual pro­perty, and parti­cu­larly scien­tists, physi­cists, mathe­ma­ti­cians, and engi­neers who could be use­ful in invig­or­ating their own atomic research pro­gram in a super­secret insti­tute simply called Laboratory Number 2 outside Moscow.

On this date, April 25, 1945, the Soviets captured Berlin’s Dahlem sub­urb and with it the Kai­ser Wil­helm Insti­tute for Physics. Cap­turing the Kaiser Wil­helm Insti­tute intact was enorm­ously import­ant to the Soviets, who knew of the insti­tute’s repu­ta­tion and that it would soon be located in the future Amer­i­can occu­pa­tion zone. As luck would have it, the Nazis had dis­persed much of the insti­tute’s equip­ment and per­son­nel to the edge of the Black Forest in South­western Germany, which lay astride the Western Front. This move allowed Allied mili­tary offi­cers, intel­li­gence per­son­nel, and scien­tific and tech­ni­cal special­ists in the secret Alsos Mission (Oper­a­tion Alsos), part of the U.S. Man­hat­tan Pro­ject, to follow close behind front lines, and occa­sion­ally behind enemy lines, and to take into cus­tody most of Ger­many’s senior nuclear research per­son­nel and tech­ni­cians, send dis­mantled equip­ment and reams of docu­ments back to the U.S. for evalu­ation, and pre­vent scien­tific assets from falling into Soviet hands. With a nuclear research pro­gram glass half-­empty, as it were, the Soviets managed to eva­cu­ate a large number of KWIP scien­tists from Berlin, along with 250 kilo­grams of metal­lic ura­nium, three tons of ura­nium oxide, and 20 liters of heavy water, to the Soviet Union, giving their nuclear program an important boost.

Alsos Mission: U.S. and British Intelligence-Gathering Operation, Late 1943 to October 1945

Alsos Mission: Haigerloch nuclear reactor, Baden-Wuerttemberg, GermanyHaigerloch nuclear reactor replica

Left: The objective of the Alsos Mission was to secure atomic mate­rial and cap­ture the scien­tists working on the Nazi atomic wea­pons pro­ject. British and Amer­i­can mem­bers of the mis­sion are pictured here dis­mantling the exper­i­men­tal nuclear reactor that German scien­tists had built as part of Germany’s nuclear energy pro­ject in Hai­ger­loch in the south­western state of Baden-Wuert­tem­berg. The reactor, shaped like a cylin­der and made of graphite blocks, was missing both uranium and heavy water.

Right: Replica of the German experi­men­tal nuclear reactor at Hai­ger­loch Museum. Hanging above the reactor are strings of uranium cubes.

Alsos Mission: Uncovering uranium cubes in Haigerloch, GermanyCol. Borish Pash, military head of Alsos Mission

Left: Members of the Alsos Mission un­cover ura­nium cubes hidden in a field in Haigerloch, which is 35 miles southwest of Stuttgart.

Right: The American sweep of South­western Ger­many—an area that was scheduled to be occu­pied by the French—was led by Col. Borish Pash (right in photo). The sweep denied the French the imme­diate post­war ad­van­tage that accrued to the U.S. and British nuclear weapons programs.

The Alsos Mission: Discovering the Extent of Germany’s Nuclear Weapons Program