In September 1915, Tsar Nicholas II assumed supreme command of the Russian Army fighting on the Eastern Front. This linked him to the country's military failures and during 1917 there was a strong decline support for his government. The country's incompetent and corrupt system could not supply the necessary equipment to enable the Russian Army to fight a modern war. By 1917 over 1,300,000 men had been killed in battle, 4,200,000 wounded and 2,417,000 had been captured by the enemy.

The High Command of the Russian Army now feared a violent revolution and on 28th February suggested that Nicholas II should abdicate in favour of a more popular member of the royal family. Attempts were now made to persuade Grand Duke Michael Alexandrovich to accept the throne. He refused and on the 1st March, 1917, the Tsar abdicated leaving the Provisional Government in control of the country. Radek joined Lenin and 26 other Bolsheviks in the sealed German train which took them to Russia.

After the October Revolution Radek became a member of the Bolshevik Central Committee. He was initially a supporter of Leon Trotsky and argued that the the Soviet government should help the spread of world revolution. In 1918 he was sent to Germany and with a group of radicals who had been members of the Spartacus League, including Rosa Luxemburg, Karl Liebknecht, Leo Jogiches, Paul Levi, Ernest Meyer, Franz Mehring and Clara Zetkin, helped to establish the German Communist Party (KPD).

Robert Bruce Lockhart, Head of Special Mission to the Soviet Government, met Radek in 1918: "A Jew, whose real name is Sobelsohn, he was in some respects a grotesque figure. A little man with a huge head, protruding ears, clean shaven face (in those days he did not wear that awful fringe which now passes for a beard), with spectacles and a large mouth with yellow, tobacco stained teeth, from which a huge pipe or cigar was never absent, he was always dressed in a quaint drab-coloured Norfolk suit with knickers and leggings. He was a great friend of Ransome, and through Ransome we came to know him very well. Almost every day he would turn up in my rooms, an English cap stuck jauntily on his head, his pipe puffing fiercely, a bundle of books under his arm, and a huge revolver strapped to his side. He looked like a cross between a professor and a bandit. Of his intellectual brilliance, however, there was no doubt. He was the virtuoso of Bolshevik journalism, and his conversation was as sparkling as his leading articles."

Radek was the Bolshevik chief of Western propaganda. In this position he developed a close relationship with Arthur Ransome of the Daily News. Ransome argued in his autobiography: "Radek had been born in Poland and spoke Polish (badly as his wife used to say, because he had talked too much German in exile), Russian (with a remarkably Polish accent) and French with the greatest difficulty. He always talked Russian with me but loved to drag in sentences from English books, which I sometimes annoyed him by being slow to recognize.... He had an extraordinary memory and an astonishingly detailed knowledge of English politics." Ransome's biographer, Roland Chambers, points out: "There was no greater linguist in the party, and no sharper wit. Sly, ambitious and mercurial, he was the Bolshevik Puck or Rumpelstiltskin, a spirit of pure mischief, and Ransome adored him.... In Radek, Ransome found an alter ego. They were like a pair of brothers, both writers, both voracious readers, both self-proclaimed bohemians with a mortal scorn for flat-footed bureaucracy."

Raymond Gram Swing of the Chicago Daily News, described Radek as "a fully seasoned conspiratorial Communist". He added: "Radek was a sharp-faced, bespectacled journalist and had a profound interest in what was happening everywhere. He had the talent I have encountered in one or two other Soviet journalists of being able to construct the news behind the news. He could read a communique and tell from the language that was used, or from what was said or omitted, just which faction or person in the Foreign Office of a government had prevailed over some other faction or individual. He may have been able to do this because Communist agents reporting on the differences between elements in government offices had supplied the background information. But he remembered it, and used it. It was a kind of scrutiny which I do not believe many United States diplomatic representatives applied to official statements in foreign countries. This faculty of Radek's greatly impressed me."

Communists were heavily involved in the German Revolution that began on 29th October 1918. Luxemburg, Liebknecht and Jogiches played a prominent role in the Spartakist Rising in Berlin. After the assassination of Kurt Eisner, in Munich on 21st February, 1919, another communist, Eugen Levine, became leader of the Bavarian Socialist Republic. The revolution was crushed by the Freikorps and its leaders were executed. Radek, together with the Comintern member Dmitry Manuilsky, made an unsuccessful attempt to launch a second German revolution in October 1923.

Victor Serge wrote in Memoirs of a Revolutionary: "Karl Radek was a sparkling writer, with an equal flair for synthesis and for sarcasm. Thin, rather small, nervous, full of anecdotes which often had a savage side to them, realistic to the point of cruelty, he had a beard growing in a fringe around his clean-shaven face, just like an old-time pirate. His features were irregular, and thick tortoise-shell spectacles ringed his myopic eyes. His walk, staccato gestures, prominent lips, and crewed-up face."

Under pressure from Lenin, Radek ceased to advocate world revolution but after the death of his leader, he supported Leon Trotsky against Joseph Stalin. In 1927 he was expelled from the party but after making public statements admitting to his "political errors" he was readmitted in 1929.

The first of what became known as show trials took place in August 1936, when Lev Kamenev, Gregory Zinoviev, Ivan Smirnov and thirteen other party members who had been critical of Stalin appeared in court. Yuri Piatakov accepted the post of chief witness "with all my heart." Max Shachtman pointed out: "The official indictment charges a widespread assassination conspiracy, carried on these five years or more, directed against the head of the Communist party and the government, organized with the direct connivance of the Hitler regime, and aimed at the establishment of a Fascist dictatorship in Russia. And who are included in these stupefying charges, either as direct participants or, what would be no less reprehensible, as persons with knowledge of the conspiracy who failed to disclose it?"

Soon after their execution, Piatakov was himself arrested. In January, 1937, Piatakov, Radek, Grigori Sokolnikov, and fifteen other leading members of the Communist Party were put on trial. They were accused of working with Leon Trotsky in an attempt to overthrow the Soviet government with the objective of restoring capitalism. Robin Page Arnot, a leading figure in the British Communist Party, wrote: "A second Moscow trial, held in January 1937, revealed the wider ramifications of the conspiracy. This was the trial of the Parallel Centre, headed by Piatakov, Radek, Sokolnikov, Serebriakov. The volume of evidence brought forward at this trial was sufficient to convince the most sceptical that these men, in conjunction with Trotsky and with the Fascist Powers, had carried through a series of abominable crimes involving loss of life and wreckage on a very considerable scale."

One of the journalists covering the trial, Lion Feuchtwanger, commented: "Those who faced the court could not possibly be thought of as tormented and desperate beings. In appearance the accused were well-groomed and well-dressed men with relaxed and unconstrained manners. They drank tea, and there were newspapers sticking out of their pockets... Altogether, it looked more like a debate... conducted in conversational tones by educated people. The impression created was that the accused, the prosecutor, and the judges were all inspired by the same single - I almost said sporting - objective, to explain all that had happened with the maximum precision. If a theatrical producer had been called on to stage such a trial he would probably have needed several rehearsals to achieve that sort of teamwork among the accused."

Piatakov and twelve of the accused were found guilty and sentenced to death. Radek and Grigori Sokolnikov were sentenced to ten years. Feuchtwanger commented that Radek "gave the condemned men a guilty smile, as though embarrassed by his luck." Maria Svanidze, who was later herself to be purged by Stalin wrote in her diary: "They arrested Radek and others whom I knew, people I used to talk to, and always trusted.... But what transpired surpassed all my expectations of human baseness. It was all there, terrorism, intervention, the Gestapo, theft, sabotage, subversion.... All out of careerism, greed, and the love of pleasure, the desire to have mistresses, to travel abroad, together with some sort of nebulous prospect of seizing power by a palace revolution. Where was their elementary feeling of patriotism, of love for their motherland? These moral freaks deserved their fate.... My soul is ablaze with anger and hatred. Their execution will not satisfy me. I should like to torture them, break them on the wheel, burn them alive for all the vile things they have done."

Karl Radek died in prison on 19th May, 1939. At first it was reported that he had been killed in a fight with a fellow inmate. However, it later emerged that he was murdered by a member of NKVD on the orders of Lavrenti Beria.