He was a leading propagandist for the Communist Party of Germany (KPD) during the Weimar Era, but later grew disenchanted with the USSR due to Joseph Stalin's Great Purge of the 1930s.

Condemned by Stalin to be purged and arrested for treason,[2] Münzenberg left the KPD and in Paris became a leader of the German émigré anti-fascism and anti-Stalinist community until forced to flee the Nazi advance into France in 1940.

His first major success was an effort to raise money and food for the victims of the Russian famine of 1921, a task which had been entrusted to him by Lenin after he was removed from the leadership of the Communist Youth International.

[5] Münzenberg was reputed to have raised millions of dollars for aid to the Soviet Union during the famine through his famous organization Internationale Arbeiter-Hilfe (IAH; "Workers International Relief"), based in Berlin.

[9] IAH's efforts were valuable not only for the practical help they offered in terms of famine relief, but also due to their propaganda value for the communist movement in Germany and around the world.

[5][4] In 1924 he launched Arbeiter-Illustrierte-Zeitung, an illustrated weekly which became the most widely read socialist pictorial newspaper in Germany, achieving a circulation of almost half a million.

[11] In addition, Münzenberg worked closely with the Comintern and the Soviet secret police (known as the Cheka in 1917–22 and as the OGPU in 1922–34) to advance the Communist cause internationally.

[4] After the fracturing of the KPD's leading triumvirate of Ernst Thälmann, Heinz Neumann and Hermann Remmele in 1931, Münzenberg participated in a behind-the-scenes factional struggle, allying with Neumann, Remmele and Leo Flieg to advocate a refocusing of the party's attacks away from the Social Democrats and towards the Nazi Party, in opposition to Thälmann and Walter Ulbricht.

Dimitrov, along with fellow Communists Blagoy Popov, Vasil Tanev, Ernst Torgler and Marinus van der Lubbe were arrested and tried on a charge of responsibility for the 1933 Reichstag fire.

The book also examined Van der Lubbe's mental health, disputed the forensic evidence presented regarding the fire, and discussed the Nazi regime's suppression of trade unions and artistic expression, anti-Semitic persecution, and use of torture against prisoners.

The counter-trial concluded one day before the trial began, and under the pressure of international public opinion, the court found Dimitrov, Popov, Tanev and Torgler not guilty.

[15] As he was barred from entering Britain at the time of the trial, Münzenberg went to the United States instead, where he spoke about and raised money for the campaign to free Thälmann from imprisonment.

He toured the northeastern and midwestern US in June 1934 with his wife Babette Gross, sister of author Margarete Buber-Neumann; Welsh Labour figure Aneurin Bevan; and SPD lawyer Kurt Rosenfeld.

[15] Speaking at well-attended rallies at venues like Madison Square Garden and the Bronx Coliseum, he appeared alongside Sinclair Lewis and Malcolm Cowley.

[11] Later in 1934, Münzenberg's influence reached the antipodes when his Comintern machine sent Egon Kisch to the All-Australian Conference of the Movement Against War and Fascism (an Australian Communist Party front organization).

Among the solidarity work he was involved with in the mid-1930s were the campaign to free Thälmann, the German Popular Front and organising aid for the Republicans during the Spanish Civil War.

The argument for the theory is that Philby was recruited to work for Soviet intelligence by one of the Münzenberg Trust's front organizations, the World Society for the Relief of the Victims of German Fascism based in Paris.

The visit occurred shortly after the first of the Moscow Trials, and shook his faith in Stalinism: on the same trip he was reprimanded by the Comintern's International Control Commission (ICC) for laxness in security and his political independence.

He persuaded Manuilsky to allow him to return to Paris in order to complete the solidarity work he had begun in aid of the Spanish Republicans before starting the position with the Comintern: however, he ran into further problems when attempting to leave the country, and was only given his passport and an exit visa after the intervention of Palmiro Togliatti.

[5][15] In late 1936, fellow KPD exile Walter Ulbricht urged him to take up an offer from Dimitrov, then residing in Moscow, to return there and assume other missions on behalf of the Comintern.

A final article on the disgraced propagandist in the Comintern journal Die Internationale warned, "Unser fester Wille, die Einheit unter den Antifaschistischen herzustellen, unser Gefühl der Verantwortlichkeit vor dem deutschen Volk macht es uns daher zur Pflicht, vor Münzenberg zu warnen.

The most important publication of his post-Communist Party period was Die Zukunft, a weekly whose contributors included German literary émigrés such as Alfred Döblin, Arnold Zweig, Thomas and Heinrich Mann, Franz Werfel, Ernst Toller and Lion Feuchtwanger, as well as international writers including Ignazio Silone, Aldous Huxley, François Mauriac, George Peabody Gooch, H. G. Wells, Julien Benda and Kingsley Martin, and global political and social figures such as Léon Jouhaux, Pietro Nenni, Francesco Saverio Nitti, Carlo Sforza, Clement Attlee, Georges Bidault, Jawaharlal Nehru, Norman Angell and Harold Macmillan.

[2] Münzenberg continued to work on behalf of antifascist causes throughout Western Europe, where he played a role in recruiting volunteers and acquiring Soviet arms for the International Brigades which fought for the Republican side in the Spanish Civil War.

[21] Münzenberg disappeared a few days later;[21] on 21 June he left his travelling companions to look for a car which would take him to the Gurs internment camp, where his partner was being held.