He was an active member of the Zimmerwald Movement and, after the October Revolution of 1917, Serrati led the PSI into joining the Comintern. During the Second Cogress of the Comintern held in Moscow in 1920, Serrati served on its Presiding Committee and was also elected to the Comintern Executive Committee that year. However, in 1921 he opposed the Comintern principle of breaking with the reformists and remained head of the Italian Socialist Party during the split into an Italian Communist Party.