The failure of the revolution in Germany, as well as his support for Leon Trotsky against Joseph Stalin, ultimately led to his fall from power and expulsion from the Party.

Radek was born in Lemberg, Austria-Hungary (now Lviv in Ukraine), as Karol Sobelsohn, to a Litvak (Lithuanian Jewish) family; his father, Bernhard, worked in the post office and died whilst Karl was young.

When the 1905 Russian Revolution broke out (including uprisings across the Kingdom of Poland), Radek participated as a revolutionary organiser in Warsaw, where he had responsibility for the party's newspaper Czerwony Sztandar.

[3]: 584–586 In 1912 August Thalheimer invited Radek to go to Göppingen (near Stuttgart) to temporarily replace him in control of the local SPD party newspaper Freie Volkszeitung, which had financial difficulties.

Basing his view on Theodore Rothstein, he claimed that, what he called the "Sinn Féin movement" was petit-bourgeois and that the backbone of earlier rebellions in Ireland, the peasant farmer, had been placated at the start of the century by the British government.

[2]: 87  However, he was refused entry to Russia[2]: 892  and went on to Stockholm, where he produced German-language versions of Bolshevik documents and other information translated from Russian, which he published in the journals Russische Korrespondenz-Pravda and Bote der Russischen Revolution.

[4]: 453 After being refused recognition as an official representative of the Bolshevik regime,[2]: 893  Radek and other delegates — Adolph Joffe, Nikolai Bukharin, Christian Rakovsky and Ignatov — traveled to the German Congress of Soviets.

The idea of creating an alliance of nations that had suffered from the Versailles treaty — principally Germany, Russia and Turkey — gained currency in Berlin, as a result of which Radek was allowed to receive a stream of visitors in his prison cell, including Walter Rathenau, Arthur Holitscher, Enver Pasha, and Ruth Fischer.

[2]: 893–4  It was Radek who took up the slogan of Stuttgart communists of fighting for a united front with other working-class organisations, that later formed the basis for the strategy developed by the Comintern.

[8] In mid-1923, Radek made his controversial speech 'Leo Schlageter: The Wanderer into the Void'[9] at an open session of the Executive Committee of the Communist International (ECCI).

[1]: 130 In the summer of 1925, Radek was appointed Provost of the newly established Sun Yat-Sen University[1]: 135  in Moscow, where he collected information for the opposition from students about the situation in China and cautiously began to challenge the official Comintern policy.

He said that "in the pages of Proust, the old world, like a mangy dog no longer capable of any action whatever, lies basking in the sun and endlessly licks its sores" and compared Joyce's Ulysses to "a heap of dung, crawling with worms, photographed by a cinema apparatus through a microscope.

In 1934 he was interviewed by a German politician, at which both of them deplored the hostile drift of their respective governments, and Radek made a controversial remark: "There are some fine lads in the SA and SS.

[17] According to an investigation of the Central Committee of the CPSU and the KGB after the Khrushchev Thaw, his murder was organized under the Supervision of the senior NKVD operative Pyotr Kubatkin.