He was strongly influenced by Johann Wolfgang von Goethe but progressively moved to a Marxist-Socialist outlook after a visit to the Soviet Union and the Nazis' appearance.

[3] After starting an undercover discussion group based at the Berlin Abendgymnasium, he met Harro Schulze-Boysen, who ran a similar faction.

Like numerous groups in other parts of the world, the undercover political factions led by Harnack and Schulze-Boysen later developed into an espionage network that supplied military and economic intelligence to the Soviet Union.

[14] The couple met Margaretha "Greta" Lorke, a German student of sociology,[15][16] at a Friday evening gathering organized by Commons in Madison.

[17] A friendship that lasted for many years developed between Mildred and Lorke,[16] the latter being drawn into an intimate group of Wisconsin radicals known as the Friday Niters Club.

[19] Many in the group protested the planned execution of the pair,[20] and Arvid petitioned the governor to create a committee to investigate the controversy.

[10] Lenz believed that only an alliance with the Soviet Union would relieve Germany of the constraints of the Treaty of Versailles, re-establish the country's position, and return it to great nation status.

[24] Harnack founded the Wissenschaftliche Arbeitsgemeinschaft zum Studium der sowjetischen Planwirtschaft ("Scientific Working Community for the Study of the Soviet Planned Economy"), or ARPLAN, with Lenz in 1931.

[28] At the time, the Harnacks were also members of the Association of Intellectual Workers (Bund der Geistesarbeiter), a communist front organisation.

[29] The real purpose of organisations like ARPLAN was to draw influential people who supported a pro-Soviet agenda in relation to German policy.

[32] Amongst those who attended were publishers Samuel Fischer, Ernst Rowohlt, and Heinrich Marie Ledig-Rowohlt; translator Franz Frein; physician and writer Max Mohr; authors and playwrights Adam Kuckhoff, Max Tau, Otto Zoff, and Ernst von Salomon; journalist Margret Boveri; critic Erich Franzen; and some of Mildred's students, including Friedrich Schlösinger.

[36] Even though the group travelled through Ukraine as academics and took notes, it failed to notice the famine before them;[34] when Arthur Koestler went there around the same time, he documented starving people in almost every station.

[41] After Adolf Hitler's rise to power and the Reichstag fire in early 1933, the Harnacks kept a low profile; they decided to dissolve ARPLAN in March 1933.

[43] In 1933, Harnack was appointed advisor at the Reich Ministry of Economics (Reichswirtschaftsministerium),[10] before becoming a senior civil servant (Oberregierungsrat) in 1938,[44][45] and worked on payment balances and foreign exchange questions about trade.

[46] As chief of trade policy, Harnack was part of many decision-making processes involving a very large number of people, including contacts in the German Foreign Office (Auswärtiges Amt).

[57] Harnack agreed to be an informer and was given the codename Balt, assigned a control officer, Alexander Belkin,[56] and given a mission to increase his sources by building a network of contacts.

[47] In 1937, former Prussian minister of culture and religious socialist Adolf Grimme was brought into the group[50] through Kuckhoff[65] and playwright Günther Weisenborn.

The most important of these was a small group called Gegner Kreis that was run by Harro Schulze-Boysen, a Luftwaffe lieutenant and descendant of an old German military family, who had known Harnack since 1935,[2] but was reintroduced to him sometime in late 1939 or early 1940 through Greta Kuckhoff.

[69] The Kuckhoffs had known the Schulz-Boysens since 1938, and started to engage them socially in autumn 1940 by bringing Mildred and Libertas (Harro's wife) together while on holiday in Saxony.

[70] Harnack and Schulze-Boysen were wary of meeting due but finally met in October 1940 at the house of Adam's and Greta Kuckhoff apartment at Wilhelmshöher Allee 19 in Friedenau.

[73] Several reasons were given as to why Harnack decided to become a spy, including a need for money, being ideologically driven, and possibly blackmail by Russian intelligence.

[79] In June 1941, with Harnack's approval, Korotkov delivered a wireless transmitter to Greta Kuckhoff during a meeting at an underground railway station.

[86] Members of both groups were convinced that Germany could only be liberated by the Nazis' military defeat, and that by shortening the war, millions of people could be saved.

In 1942, Harnack produced a study called "Das nationalsoialistische Stadium des Monopolkapitalismus" (The National stage of monopoly capitalism), published in Die innere Front, which described the Gestapo as a tendentious and antigovernment economic treatise, and was read as far as Munich and Hamburg.

Harnack arranged with the Communist Party in Hamburg via Bernhard Bästlein[84] to pass the reports through contacts in Flensburg and Denmark to the Soviet embassy in Stockholm.

His exposure of the radio codes enabled Referat 12, the cipher bureaux of the Funkabwehr, to decipher Red Orchestra message traffic.

The unit had been tracking Red Orchestra radio transmissions since June 1941, and found Wenzel's house in Brussels contained a large number of coded messages.

[95] When Vauck decrypted this message, it was forwarded to Reich Security Main Office IV 2A, where they identified the people living at the three addresses.

She was beheaded by guillotine, and her body was released to Hermann Stieve, anatomy professor at Humboldt University, to be dissected for research.

A cenotaph was installed for the Harnacks after the war by Arvid's older brother Falk, a member of the White Rose resistance group, at Zehlendorf Cemetery.