Zinoviev spent the first three years of World War I in Switzerland. After the Russian monarchy was overthrown during the February Revolution, he returned to Russia in April 1917 in a sealed train with Lenin and other revolutionaries opposed to the war. He remained a part of the Bolshevik leadership throughout most of that year and spent time with Lenin after Lenin was forced into hiding after an abortive coup attempt in July 1917. However, Zinoviev and Lenin soon had a falling out over Zinoviev's opposition to the Bolshevik seizure of power in October. On October 10, 1917 (Julian calendar), he and Lev Kamenev were the only two Central Committee members to vote against an armed revolt. Their publication of an open letter opposed to use of force enraged Lenin, who demanded their expulsion from the party.

On October 29, 1917 (Julian calendar), immediately after the Bolshevik seizure of power during the October Revolution, the executive committee of the national railroad labor union, Vikzhel, threatened a national strike unless the Bolsheviks shared power with other socialist parties and dropped Lenin and Leon Trotsky from the government. Zinoviev, Kamenev, and their allies in the Bolshevik Central Committee argued that the Bolsheviks had no choice but to start negotiations since a railroad strike would cripple their government's ability to fight the forces that were still loyal to the overthrown Provisional Government. Although Zinoviev and Kamenev briefly had the support of a Central Committee majority and negotiations were started, a quick collapse of the anti - Bolshevik forces outside Petrograd allowed Lenin and Trotsky to convince the Central Committee to abandon the negotiating process. In response, Zinoviev, Kamenev, Alexei Rykov, Vladimir Milyutin, and Victor Nogin resigned from the Central Committee on November 4, 1917 (Julian calendar). The following day, Lenin wrote a proclamation calling Zinoviev and Kamenev "deserters". He never forgot their behavior, eventually making an ambiguous reference to their "October episode" in his Testament.

This was an important turning point in Zinoviev's career. For the next five years it was Trotsky and not Zinoviev who was the number two man in the Bolshevik Party. Zinoviev, an ambitious man, was not willing to accept his demotion and did much to undermine Trotsky's position within the Party between 1918 and 1925.

Zinoviev soon returned to the fold and was once again elected to the Central Committee at the VII Party Congress on March 8, 1918. He was put in charge of the Petrograd (Saint Petersburg prior to 1914, Leningrad 1924 - 1991) city and regional government. In September 1918 he declared:

To overcome of our enemies we must have our own socialist militarism. We must carry along with us 90 million out of the 100 million of Soviet Russia's population. As for the rest, we have nothing to say to them. They must be annihilated.

He became a non - voting member of the ruling Politburo when it was created after the VIII Congress on March 25, 1919. He also became the Chairman of the Executive Committee of the Comintern when it was created in March 1919. It was in this capacity he presided over the Congress of the Peoples of the East in Baku in September 1920 and gave his famous four hour speech in German at the Halle congress of the Independent Social Democratic Party of Germany in October 1920.

Zinoviev was responsible for Petrograd's defense during two periods of intense clashes with White forces in 1919. Trotsky, who was in overall charge of the Red Army during the Russian Civil War, thought little of Zinoviev's leadership, which aggravated their strained relationship.

In early 1921, when the Communist Party was split into numerous factions and disagreements were threatening to engulf the Party, Zinoviev supported Lenin's faction. As a result, Zinoviev was made a full member of the Politburo after the Xth Party Congress on March 16, 1921, while members of other factions such as Nikolai Krestinsky were dropped from the Politburo and the Secretariat.

Zinoviev was one of the most powerful figures in the Soviet leadership during Lenin's final illness in 1922 - 1923 and immediately after his death in January 1924. He delivered the Central Committee's reports to the XIIth and XIIIth Party Congresses in 1923 and 1924 respectively, something that Lenin had previously done. He was also considered one of the Communist Party's leading theoreticians. As head of the Comintern, Zinoviev deserved some of the blame for the failure of the Communist revolt in Germany in the fall of 1923, but he managed to shift it to Karl Radek, the Comintern's representative in Germany in 1923.

During Lenin's illness, Zinoviev, his close associate Kamenev, and Joseph Stalin formed a ruling 'triumvirate' (or 'troika') in the Communist Party, playing a key role in the marginalization of Leon Trotsky. The triumvirate carefully managed the intra - party debate and delegate selection process in autumn 1923 during the run up to the XIIIth Party Conference and secured a vast majority of the seats. The Conference, held in January 1924 just before Lenin's death, denounced Trotsky and Trotskyism. Some of Trotsky's supporters were demoted or reassigned in the wake of his defeat, and Zinoviev's power and influence appeared to be at its zenith. However, as subsequent events showed, his real power base was limited to the Petrograd / Leningrad Party organization while the rest of the Communist Party apparatus was increasingly under Stalin's control.

After Trotsky's defeat at the XIIIth Conference, tensions between Zinoviev and Kamenev on the one hand and Stalin on the other hand became more pronounced and threatened to end their fragile alliance. Nevertheless, Zinoviev and Kamenev helped Stalin retain his position as General Secretary of the Central Committee at the XIIIth Party Congress in May - June 1924 during the first Lenin's Testament controversy.

After a brief lull in the summer of 1924, Trotsky published The Lessons of October, an extensive summary of the events of 1917. In the article, Trotsky described Zinoviev's and Kamenev's opposition to the Bolshevik seizure of power in 1917, something that the two would have preferred to be left unmentioned. This started a new round of intra - party struggle with Zinoviev and Kamenev once again allied with Stalin against Trotsky. They and their supporters accused Trotsky of various mistakes and worse during the Russian Civil War and damaged his military reputation so much that he was forced to resign as People's Commissar of Army and Fleet Affairs and Chairman of the Revolutionary Military Council in January 1925. Zinoviev demanded Trotsky's expulsion from the Communist Party, but Stalin refused to go along at that time and skillfully played the role of a moderate.

With Trotsky finally on the sidelines, the Zinoviev - Kamenev - Stalin triumvirate began to crumble in early 1925. The two sides spent most of the year lining up support behind the scenes. Stalin struck an alliance with Communist Party theoretician and Pravda editor Nikolai Bukharin and Soviet prime minister Alexei Rykov. Zinoviev and Kamenev allied with Lenin's widow, Nadezhda Krupskaya, and Grigory Sokolnikov, the Soviet Commissar of Finance and non - voting Politburo member. The struggle became open at the September 1925 meeting of the Central Committee and came to a head at the XIVth Party Congress in December 1925. With only the Leningrad delegation behind them, Zinoviev and Kamenev found themselves in a tiny minority and were soundly defeated. Zinoviev was re-elected to the Politburo, but his ally Kamenev was demoted from a full member to a non - voting member and Sokolnikov was dropped altogether, while Stalin had more of his allies elected to the Politburo. Within weeks of the Congress, Stalin wrestled control of the Leningrad party organization and government from Zinoviev and had him dismissed from all regional posts, leaving only the Comintern as a potential power base for Zinoviev.

During a lull in the intra - party fighting in the spring of 1926, Zinoviev, Kamenev and their supporters gravitated closer to Trotsky's supporters and the two groups soon formed an alliance, which also incorporated some smaller opposition groups within the Communist Party. The alliance became known as the United Opposition. In May 1926, Stalin, weighing his options in a letter to Vyacheslav Molotov, directed his supporters to concentrate their attacks on Zinoviev since the latter was intimately familiar with Stalin's methods from their time together in the triumvirate. Following Stalin's orders, his supporters accused Zinoviev of using the Comintern apparatus in support of factional activities (the Lashevich Affair) and Zinoviev was dismissed from the Politburo after a tumultuous Central Committee meeting in July 1926. Soon thereafter the office of the Comintern Chairman was abolished and Zinoviev lost his last important post.

Zinoviev remained in opposition to Stalin throughout 1926 and 1927, resulting in his expulsion from the Central Committee in October 1927. When the United Opposition tried to organize independent demonstrations commemorating the 10th anniversary of the Bolshevik seizure of power in November 1927, the demonstrators were dispersed by force and Zinoviev and Trotsky were expelled from the Communist Party on November 12. Their leading supporters, from Kamenev down, were expelled in December 1927 by the XVth Party Congress, which paved the way for mass expulsions of rank and file oppositionists as well as internal exile of opposition leaders in early 1928.

While Trotsky remained firm in his opposition to Stalin after his expulsion from the Party and subsequent exile, Zinoviev and Kamenev capitulated almost immediately and called on their supporters to follow suit. They wrote open letters acknowledging their mistakes and were readmitted to the Communist Party after a six month cooling off period. They never regained their Central Committee seats, but they were given mid - level positions within the Soviet bureaucracy. Bukharin, then at the beginning of his short and ill fated struggle with Stalin, courted Kamenev and, indirectly, Zinoviev during the summer of 1928. This was soon reported to Stalin and used against Bukharin as proof of his factionalism.

Zinoviev and Kamenev remained politically inactive until October 1932 when they were expelled from the Communist Party for failure to inform on oppositionist party members during the Ryutin Affair. After once again admitting their mistakes, they were readmitted in December 1933. They made self flagellating speeches at the XVIIth Party Congress in January 1934 when Stalin was parading his erstwhile political opponents, now defeated and outwardly contrite.

After the murder of Sergei Kirov on December 1, 1934, which served as one of the triggers to Great Purge of the Communist Party, Zinoviev, Kamenev, and their closest associates were once again expelled from the party and arrested in December 1934. They were tried in January 1935 and were forced to admit "moral complicity" in Kirov's assassination. Zinoviev was sentenced to 10 years in prison and his supporters to various prison terms.

In August 1936, after months of careful preparations and rehearsals in Soviet secret police prisons, Zinoviev, Kamenev, and 14 others, mostly Old Bolsheviks, were put on trial again. This time, the charges included forming a terrorist organization that supposedly killed Kirov and tried to kill Stalin and other leaders of the Soviet government. This Trial of the Sixteen (or the trial of the "Trotskyite - Zinovievite Terrorist Center") was the first Moscow Show Trial and set the stage for subsequent show trials where Old Bolsheviks confessed to increasingly elaborate and monstrous crimes, including espionage, poisoning, sabotage, and so on. Zinoviev and the other defendants were found guilty on August 24, 1936.

Prior to the trial, Zinoviev and Kamenev had agreed to plead guilty to the false charges on the condition that they not be executed, a condition that Stalin accepted, stating "that goes without saying." Nonetheless, a few hours after the conviction, Stalin ordered their execution for that night. Shortly after midnight, on the morning of August 25, Zinoviev and Kamenev were executed by shooting.

Accounts of Zinoviev's execution vary, with some having him beg and plead for his life, prompting a more stoic Kamenev to tell Zinoviev to quiet down and die with dignity. Regardless, Zinoviev put up such resistance against the guards that, instead of taking him to the appointed execution room, the guards took him into a nearby cell and shot him there.

The execution of Zinoviev, Kamenev, and their associates was a sensational news event in the USSR and around the world, paving the way for the mass arrests and executions of the terror of 1937 - 1938. Zinoviev and his co-defendants were formally absolved of the spurious charges which led to their death by the Soviet government in 1988 during perestroika.

Zinoviev is remembered in Britain as the alleged author of the "Zinoviev Letter" which caused a sensation when published on October 25, 1924, four days before a general election. The letter called on British Communists to prepare for revolution. This document is now generally accepted to have been a forgery, validating the declaration that Zinoviev made in a letter dated 27 October 1924:

"The letter of 15th September, 1924, which has been attributed to me, is from the first to the last word, a forgery. Let us take the heading. The organization of which I am the president never describes itself officially as the Executive Committee of the Third Communist International; the official name is Executive Committee of the Communist International. Equally incorrect is the signature, The Chairman of the Presidium. The forger has shown himself to be very stupid in his choice of the date. On the 15th of September, 1924, I was taking a holiday in Kislovodsk, and, therefore, could not have signed any official letter.... "It is not difficult to understand why some of the leaders of the Liberal - Conservative bloc had recourse to such methods as the forging of documents. Apparently they seriously thought they would be able, at the last minute before the elections, to create confusion in the ranks of those electors who sincerely sympathize with the Treaty between England and the Soviet Union. It is much more difficult to understand why the English Foreign Office, which is still under the control of the Prime Minister, MacDonald, did not refrain from making use of such a white - guardist forgery."