 After his return, he led the left-wing of Social Democracy, preparing for the creation of the communist party and participated in work of the Comintern.In May 1921 he co-founded, together with Antonín Zápotocký, the Communist Party of Czechoslovakia (KSČ) and served in its executive committee (later named the central committee, ÚV KSČ).Šmeral was criticized as too moderate and for "social democratism" by radicals.