The first two decades of the PSI were marked by an internal struggle led by the left faction to establish Marxism as the party's official ideology.
By 1912, this faction had become dominant within the PSI, with key figures including Benito Mussolini, Angelica Balabanoff, and Amadeo Bordiga.
Mussolini broke with the left by adopting a pro-Allied stance, while Bordiga developed an anti-war position similar to Lenin's revolutionary defeatism.
The implicit support given by the Comintern at the 2nd World Congress enabled the Abstentionist Communist Faction to break out of its isolation as a minority in the party.
Initially close to Serrati's maximalists and in favor of participating in elections, they first entered into polemics with the Abstentionist Communist Faction, only to move closer to it in 1920 as it gained majority support in cities such as Naples, Milan, Florence, and Turin.
Despite representing the majority faction, the PCd'I's left leadership was replaced in 1924 by Gramsci under pressure from the Comintern, then effectively controlled by Joseph Stalin.
[12] The Left Faction of the Communist Party of Italy formed in 1928, primarily composed of Italian émigré communities in Belgium, France, and the United States.
[13] The formation was prompted by Leon Trotsky's expulsion from the Soviet Union, the adoption of the theory of socialism in one country, as well as a growing number of other disagreements with Comintern policies.
[17] In 1943, a nucleus led by militants including Onorato Damen, Fausto Atti, Mario Acquaviva, and Bruno Maffi established the Internationalist Communist Party in Northern Italy.
The faction centered around Damen favored electoral participation and rejected both union work and national liberation struggles, whereas the faction centered around Bordiga opposed the policy of revolutionary parliamentarianism, supported union work, and maintained the Communist International's position on national and colonial questions.