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 (which the Germans called a Ura­nium Machine), 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, a process required to pre­pare enriched ura­nium for use in nuclear weapons. 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 relocated in the French-des­ig­nated 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 build the world’s first nuclear bomb, 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. These scien­tific commandos, who referred to them­selves as the “bastard bri­gade” because they worked inde­pen­dently, unattached to any larger mili­tary group, sent dis­mantled equip­ment and reams of docu­ments back to the U.S. for eval­u­a­tion and pre­vented high-value scien­tific assets from falling into Soviet (or French) 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. As part of Oper­a­tion Big Bang, British and Amer­i­can mem­bers of the mis­sion are pictured here dis­mantling the exper­i­men­tal nuclear reactor that German scien­tists and tech­ni­cians had built as part of their coun­try’s nuclear weapons pro­ject in Hai­ger­loch, a rustic vil­lage about 35 miles/­56 km south­west of Stutt­gart in the south­western state of Baden-Wuert­tem­berg. The reactor was nestled in a natural cave 25 yards/­23 meter deep at the base of an 80‑ft/24‑m-tall limestone cliff. Dubbed the “atom cellar” by Alsos team mem­bers, it con­tained the pre­cious German Ura­nium Machine in the shape of a cylin­der bored into the lime­stone floor and lined with graph­ite blocks enclosing a central alu­mi­num vat that would have been filled with heavy water; the reactor was missing both ura­nium and heavy water at the time of its dis­covery. The Germans had evac­u­ated and dis­mantled their nuclear reactor before Alsos’s arrival.

Right: Replica of the German experi­men­tal nuclear reactor at Hai­ger­loch Museum. Hanging above the reactor’s alu­mi­num vat are strings of fake ura­nium cubes. In late March 1945 German scien­tists came close to creating a working nuclear reactor when they lowered hun­dreds of ura­nium cubes into Hai­ger­loch’s alu­mi­num vat filled with heavy water—this 28 months after Enrico Fermi and his team of Amer­i­can scien­tists had demon­strated the first sus­taining nuclear reactor at the Uni­versity of Chicago on Decem­ber 2, 1942. Alsos dealt the Nazi atomic bomb pro­ject a mortal blow by setting a charge of dyna­mite that collapsed the roof of the atom cellar, liter­ally bringing the cur­tain down on the last lingering remnants of the German “Manhattan Project.”

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

Left: Once the atom cellar was buried under tons of lime­stone rock, mem­bers of the Alsos Mis­sion set out lickety-split to capture the brains behind the Hai­ger­loch reactor, any asso­ci­ated tech­ni­cal docu­men­ta­tion, and the missing ura­nium and heavy water. Quickly, 3,000 lb/­1.36 metric tons of heavy water were recovered in oil drums hidden in a grist mill 3 miles/­4.82 km distant; the tech­ni­cal docu­men­ta­tion—indeed, prac­ti­cally the whole archive of the Nazi atomic bomb project—was flushed out of a cess­pool. Sim­i­larly, two tons of ura­nium in the form of hun­dreds of 2-inch/­5-cm cubes—the vast major­ity of the Nazis’ ura­nium horde—were recovered in a field atop a nearby hill, as seen in this photo. Fearing booby traps, Alsos forced German POWs to dig up the cubes, some of which were kept as souvenirs by the Alsos scientists.

Right: The American sweep of Southwestern Germany—an area that was scheduled to be occu­pied by the French—was led by the hard-charging World War I veteran Col. Boris 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