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A London crossing in 1933

After reading a newspaper report of Ernest Rutherford's warning that practical power from atomic transmutation was "moonshine," Leó Szilárd pondered the problem while walking in London in 1933.

By his own later recollection, he stopped at a red light on Southampton Row; as he crossed after the light changed, he conceived the condition for a chain reaction — a neutron-triggered nucleus would have to release multiple neutrons capable of repeating the reaction. One absorbed neutron would have to produce two.

An idea five years early

The insight was conceptual, not an experiment or a public announcement. Szilárd did not yet know an element that could sustain the reaction, and nuclear fission had not been discovered.

James Chadwick's discovery of the neutron the year before had supplied the electrically neutral particle at the centre of the proposed mechanism.

Nothing observable changed in London that day. Over the following months Szilárd discussed candidate elements, pursued experiments and filed a British chain-reaction patent application on 28 June 1934, later published as GB630726A.

The date and the corner are both reconstructions

12 September is an inference, not a date Szilárd recorded at the time; Richard Rhodes derived it because the relevant Times report appeared that morning and Szilárd said he read it before his walk.

Szilárd's own archival finding aid dates the insight to October 1933, while other accounts say September.

The exact crossing is uncertain too — later accounts variously place it near Russell Square or High Holborn, and the UC San Diego archive says the precise point is unknown.

5 March 1933, under the fire decree

German voters elected a new Reichstag on 5 March 1933, while Hitler was already chancellor and the Reichstag Fire Decree had suspended core civil liberties. Opposition politicians and publications — Communist ones particularly — had been arrested, banned or driven underground.

In Prussia, 50,000 armed SA, SS and Stahlhelm members had been deputized as auxiliary police. A contemporary US diplomatic report concluded that Nazi terrorism and the suspension of elementary liberties had deprived the election of the character of a free expression of the popular will.

43.9 percent, and the arithmetic beneath it

The NSDAP received 17,277,180 votes — 43.9 percent — and 288 of 647 seats, making it the largest party but leaving it short of an outright majority. Turnout was 88.7 percent.

The SPD took 18.3 percent, the KPD 12.3 percent and 81 seats. The NSDAP's existing coalition partner, the DNVP-led Kampffront Schwarz-Weiß-Rot, held 52 seats, which gave the government 340 and a working majority.

What the government did with the result

It was not the two-thirds constitutional majority Hitler wanted. Within three days the regime annulled the newly elected KPD mandates, altering the Reichstag the vote had produced.

On 23 March, arrests, exclusions, threats and Centre Party support carried the Enabling Act.

15 February 1935, in a single journal issue

On 15 February 1935, issue 7 of volume 61 of Deutsche Medizinische Wochenschrift published Gerhard Domagk's “Ein Beitrag zur Chemotherapie der bakteriellen Infektionen” at pages 250–253. He reported that Prontosil protected streptococcus-infected mice whether given orally or by injection, even though the compound had shown little useful activity in a test tube.

In the starkest experiment, 26 mice were infected: all 12 treated animals survived, and all 14 untreated controls were dead by day four.

Bedside reports on the following pages

The journal placed clinical reports directly after the experimental paper. Philipp Klee and Herbert Römer reported treatment of streptococcal disease at pages 253–255, and H. T. Schreus reported erysipelas and other infections at 255–256.

Laboratory evidence and physicians' results ran back to back across seven pages.

Why it surfaced when it did

Prontosil had been synthesized in 1932 by Josef Klarer and Fritz Mietzsch, within an industrial azo-dye program at IG Farben's Bayer laboratories that had produced more than 300 azo compounds by that year.

IG Farben kept the work largely inside the company while patenting and clinical investigation proceeded, and its German patent was granted in December 1934. The date marks publication and public clinical introduction — not the first synthesis, the first animal test or the first administration to a patient.

15 September 1935: a special session at the party rally

At a special night session during the Nazi Party rally in Nuremberg on 15 September 1935, the all-Nazi Reichstag unanimously adopted three laws: the Reich Citizenship Law, the Law for the Protection of German Blood and German Honor, and the Reich Flag Law.

Hitler had originally summoned the deputies to enact a flag law. Amid demands for anti-Jewish legislation at the rally, he ordered Interior Ministry lawyers to draft a Blood Protection Law and then a Citizenship Law within roughly two days.

Hermann Göring, as president of the Reichstag, read the legislation to the assembled deputies.

What the two racial laws actually said

The Reich Citizenship Law limited Reich citizenship to state subjects of "German or kindred blood" who met the regime's conduct test. Only Reich citizens were to possess full political rights.

The Blood Protection Law prohibited marriages and extramarital sexual relations between Jews and citizens of German or related blood, restricted Jewish households' employment of certain women, and barred Jews from flying the Reich or national flag.

The household provision applied to female citizens of German or related blood under 45.

The definition arrived two months later

The laws were published in the Reich Law Gazette the following day. Wilhelm Stuckart and Bernhard Lösener were among the Interior Ministry officials who did the hurried drafting in Nuremberg immediately before the session.

Neither September statute contained a genealogical formula for who counted as a Jew. That machinery came on 14 November, when an implementing regulation classified people through the religious affiliation of their grandparents — at least three grandparents classified as fully Jewish, with separate rules for people with two — and declared that a Jew could not be a Reich citizen.

Enforcement did not wait for the 1936 Olympics: a scholarly tally records 11 convictions for "racial defilement" during 1935.

9 November 1938: a death in Paris, a decision in Munich

At approximately 5:30 p.m. on 9 November 1938, Nazi leaders gathered in Munich learned that the diplomat Ernst vom Rath had died of wounds inflicted two days earlier by Herschel Grynszpan, a seventeen-year-old Polish Jewish refugee.

After conferring, Hitler and Joseph Goebbels decided to instigate a nationwide anti-Jewish pogrom. Around 9:30–10 p.m., Goebbels addressed the Nazi dignitaries, who telephoned instructions to party districts.

An internal Nazi Party report later acknowledged that officials understood the Party should conceal its role while actually organising and executing the "demonstrations".

Orders before midnight, and again at 1:20 a.m.

Before midnight, Gestapo chief Heinrich Müller ordered the arrest of 20,000–30,000 Jewish men. Through the night, SA, SS, Hitler Youth and other participants attacked Jewish people, homes, businesses and synagogues across Germany and its annexed territories.

At 1:20 a.m. on 10 November, Reinhard Heydrich ordered police not to hinder the violence, to protect non-Jewish property, and to arrest healthy Jewish men, especially affluent ones.

Firefighters were ordered to let synagogues burn and to intervene only if neighbouring property was endangered.

The pogrom's documented toll

By the morning of 10 November, synagogues were burning, Jewish homes and shops had been wrecked, and police were carrying out mass arrests. More than 1,400 synagogues were destroyed or severely damaged, and roughly 7,500 Jewish-owned businesses had their windows smashed and their goods looted. About 26,000 Jewish men were imprisoned in Dachau, Buchenwald and Sachsenhausen.

The regime formally called off the riot at 4 p.m. on 10 November, though some violence continued afterward. The Jewish community was then fined 1 billion Reichsmarks.

The often-repeated figure of 91 dead counts only those recorded as murdered during the pogrom itself; broader research, including deaths from wounds, medical crises, suicide and camp maltreatment, reaches hundreds.

27 August 1939: a circuit over Rostock-Marienehe

On 27 August 1939, Erich Warsitz flew the experimental Heinkel He 178 at the Heinkel works airfield at Rostock-Marienehe. Powered by Hans von Ohain's HeS 3B engine, it became the first aircraft to fly using turbojet thrust.

The prototype had a duralumin fuselage, wooden wings and a nose intake feeding its internally mounted engine. A German public-broadcast history describes the machine as about 7.5 metres long and just under two tonnes.

Warsitz completed a circuit and landed safely.

The gear stayed down, and the clock is disputed

Warsitz's retrospective account, published by his son, says the retractable landing gear would not retract and remained down.

Published accounts also disagree on how long the flight lasted: one secondary account gives six minutes, and the German broadcast history gives about eight.

Several secondary sources report a brief hop on 24 August, but the better-supported milestone is the sustained flight on the 27th.

Nobody official was watching

The flight was conducted secretly, and senior Luftwaffe officials did not witness it. The demonstration attended by Ernst Udet and Erhard Milch came later, on 1 November.

Von Ohain's earlier HeS 3A had lacked sufficient thrust; the improved HeS 3B was installed in the He 178 in early August 1939 and made a powered flight practicable. A Smithsonian history reports it at about 1,100 pounds of thrust.

The He 178 was an experimental proof of concept, not an operational fighter, and it never entered service.

December 1940: 16 MeV deuterons on uranium

Glenn Seaborg, Joseph Kennedy and Arthur Wahl bombarded uranium with 16 MeV deuterons in Berkeley's 60-inch cyclotron on 14 December 1940. The neptunium-238 it produced beta-decayed into plutonium-238, and the alpha activity growing in the target was what the three suspected was element 94.

Edwin McMillan's discovery of neptunium with Philip Abelson had implied that an element with atomic number 94 should exist. McMillan left Berkeley for radar work in November 1940, having given Seaborg his assent to continue the search, and stated that he did not participate in the final chemical proof.

A suspicion was all it was. Producing the material settled nothing about whether it was a new element, and the question stayed open for more than two months.

The confirmation, on the night of 23–24 February 1941

During the night of 23–24 February 1941, Wahl carried out an oxidation experiment in Room 307 of Gilman Hall at the University of California, Berkeley. It separated the alpha-emitting material from neptunium and supplied chemical proof that it was a new element.

February 23 is the conventional anniversary, the one the American Chemical Society marks, while the Department of Energy sometimes lists February 24 for the same experiment. Sources also use "synthesized," "produced," "identified" and "discovered" interchangeably. The two dates are not rivals: the material was produced in December 1940 and chemically identified on the night of 23–24 February 1941.

An element studied without being seen

What the Berkeley group had was tracer quantities, followed through their radioactivity and chemical behaviour rather than looked at. Even half a microgram mixed with carrier material counted as a huge sample.

The element was named plutonium after Pluto, extending the sequence uranium–Uranus and neptunium–Neptune.

A visible amount was not isolated until 20 August 1942, and plutonium was first weighed on 10 September 1942.

The oxidation test in Room 307

During the night of 23–24 February 1941, the graduate student Arthur C. Wahl performed the decisive oxidation experiment in Room 307 of Gilman Hall at the University of California, Berkeley. It showed that the radioactive material behaved chemically unlike every known element, supporting its identification as element 94.

The team had already detected an alpha-emitting daughter product from neptunium-238, but needed chemical evidence separating it from known elements, particularly thorium. Wahl's experiment supplied that evidence.

Where the material came from

The substance was plutonium-238. It was produced after uranium was bombarded with deuterons in Berkeley's 60-inch cyclotron, creating neptunium-238 that beta-decayed into plutonium-238.

That bombardment had taken place on 14 December 1940. The February date commemorates the conclusive chemical identification, not the first nuclear production of the material.

What “isolated” overstates

This was a trace-scale chemical separation rather than the isolation of a visible piece of plutonium metal. The first visible amount was obtained in August 1942, and plutonium was first weighed that September.

The experiment also crossed midnight, which is why the record wobbles: Glenn T. Seaborg dated it to the night of 23–24 February, some timelines therefore use 24 February, and one chemical-history account describes the isolation work as spanning 23–25 February.

Seaborg, who directed the element-94 investigation, later credited the discovery to Wahl, Joseph W. Kennedy, Edwin M. McMillan and himself.

9 May 1941: forced to the surface off Greenland

While protecting Convoy OB 318 in the North Atlantic off Greenland on 9 May 1941, the corvette HMS Aubretia dropped depth charges that badly damaged U-110 and forced the German submarine to the surface.

Its commander, Fritz-Julius Lemp, ordered the crew to abandon the boat without first ensuring that its secret material and equipment were destroyed.

Accounts differ over the precise circumstances of Lemp's death after the abandonment.

Six hours inside the boat

Sub-Lieutenant David Balme, aged 20, led a boarding party across from HMS Bulldog and entered the apparently deserted submarine.

Telegraphist Allen Long found a device resembling a typewriter, pressed its keys, saw peculiar results and passed it up through the hatch. The party recovered a working Enigma machine, cipher keys, keybooks and other cryptographic records.

They worked inside the damaged boat for about six hours before preparing it for towing.

Taken, and kept quiet

The Admiralty ordered the capture treated with extreme secrecy. Bulldog attempted to tow U-110 toward Iceland, but the submarine foundered on 10 May — after its intelligence material had been removed.

The machine by itself did not suddenly teach Britain to read every German code. The current key material, operating instructions and codebooks were the particularly valuable part, building on earlier Polish and British cryptanalysis.

12 May 1941: a private audience in Kreuzberg

Konrad Zuse demonstrated the working Z3 on 12 May 1941 at Methfesselstraße 7 in Berlin-Kreuzberg, to a small invited group of experts rather than to the general public.

The audience included Alfred Teichmann and Curt Schmieden of the German Laboratory for Aviation, whose researchers were interested in automated engineering calculations.

Zuse had built the machine through his firm, Zuse-Apparatebau.

About 2,000 relays, clicking at 5 to 10 Hz

The demonstrated machine executed programs from external punched tape and performed binary floating-point arithmetic automatically with electromechanical relays — about 2,000 of them, on a 22-bit word, clocked at roughly 5 to 10 Hz.

Changing programs did not require rewiring: the instructions arrived on punched film outside the machine. It was electromechanical, not electronic.

Loops were possible only by physically joining the tape into a loop; the instruction set contained no conditional branch.

What the priority claims will and will not carry

Authoritative sources call the Z3 the first functioning, freely programmable binary computer, or one of the first working programmable fully automatic machines. Blunter labels such as "the first computer" depend entirely on definitions.

Raúl Rojas showed in 1998 that a looped tape and arithmetic simulation could make the Z3's computing model universal in principle — which is not the same as practical modern-style branching.

The demonstration was not a launch: use remained limited while Zuse moved on to the Z4 and filed a patent application for the Z3 principle on 16 June 1941.

"Ruthless action" written into troop conduct

On 4 June 1941 the Guidelines for the Conduct of the Troops in Russia reached Wehrmacht divisional commands, weeks before Operation Barbarossa opened on 22 June. The chronology is disputed: the same document carries a creation or issue date of 19 May 1941.

The text told soldiers that Germany was fighting Bolshevism and demanded ruthless action against "Bolshevik agitators, guerrillas, saboteurs, and Jews", together with the elimination of active or passive resistance.

The other named categories purported to describe conduct; Jews were listed beside them regardless of conduct. Historians interpret the wording as casting Jews collectively as racial-political enemies and legitimating military violence against them.

Why the same directive is filed under two dates

The 4 June 1941 date is the label the US Holocaust Memorial Museum attaches to its translated excerpt of the guidelines.

The German Federal Archives instead catalogue the document as annex 3 to an Armed Forces Operations Staff file created on 19 May 1941, and much of the scholarship dates it accordingly.

Christopher Browning's account of the origins of the Final Solution supports reconciling the two: 19 May for creation or formal issue, 4 June for distribution down to divisional level.

The order series it arrived with

Which headquarters issued the guidelines is not settled. The Bundesarchiv catalogue records the surviving document's provenance as the Army High Command's Generalquartiermeister branch — OKH/GenQu — while institutional histories, the US Holocaust Memorial Museum among them, place the guidelines within the Oberkommando der Wehrmacht's series of pre-invasion orders. Either way they belonged to a wider complex of criminal orders that removed legal protections from Soviet civilians and prisoners of war.

The guidelines did not themselves contain a universal, operational order to murder every Soviet Jew, although they were an important instrument of antisemitic incitement.

Divisional commands now held an antisemitic and anti-Bolshevik indoctrination document to transmit to their units. Two days after the 4 June distribution, the separate Commissar Order of 6 June explicitly directed the killing of captured Soviet political commissars.

29 September 1941: assemble with your documents

On 29 September 1941, Jewish men, women, children, elderly people and sick people obeyed posted orders to assemble near Kyiv's cemeteries with documents, money, valuables and clothing, under threat of execution for noncompliance. Two thousand wall posters had been prepared for the roundup.

German military police and Order Police directed the column toward Babyn Yar, confiscated belongings, drove victims through a gauntlet and forced them to undress before they reached the ravine.

The operation began on the eve of Yom Kippur.

In the ravine, and then overnight

Sonderkommando 4a under Paul Blobel, Einsatzgruppe C personnel and units of German Police Regiment South shot the victims in groups inside the ravine. People were made to lie on bodies before being shot, and the dead and wounded were covered with soil and other material.

When darkness fell, Jews not yet shot were confined in garages overnight. The killing resumed on 30 September.

The number the killers reported to Berlin

A contemporary Einsatzgruppen report recorded that Sonderkommando 4a, the Einsatzgruppe staff and two Police Regiment South commands killed 33,771 Jews on 29–30 September. Ammunition supplied shortly beforehand included 5,000 pistol or rifle rounds and as many as 100,000 machine-gun rounds.

This was not spontaneous retaliation for the Soviet-planted explosions and fires in occupied Kyiv. German authorities used those as a pretext and convened SS, police and Wehrmacht officials — including the city's military governor, Kurt Eberhard — to organise the killing.

Local auxiliaries helped post notices, guard and route people and handle stolen property, but scholarship cautions that the identities and precise functions of Ukrainian-speaking personnel at the ravine are not fully established.