In 1918, Walecki was expelled from Switzerland for his role in a railway workers' strike, and returned to Warsaw, where he was one of the founders of the Communist Party of Poland (KPP), formed in 1918 by a merger of the PPS-Lewica and the SDPKiL, and a member of its Central Committee in 1918–20 and 1923–24, and politburo in 1923–24.

Arrested in the winter of 1919, he was released on bail after several months, and fled to Russia, where he formed part of the triumvirate known as the 'three Ws', who led the Polish Communist Party in exile.

[4] Also, late in 1923, as Lenin's terminal illness set off a power struggle in Moscow, the KPP issued a statement defending Trotsky.

Alexander Barmine, who was based in Riga in 1922, returned to Moscow on the same train as delegates to the Fourth Congress of Comintern, including Walecki and Eugen Varga, and complained in his memoirs that they "showed the most revolting lack of consideration" by demanding that they be allocated a private compartment on a crowded train, and lodging a complaint in Moscow when he turned them down.

"[7] Walecki continued to work for Comintern, where he was deputy head of the Balkan Secretariat in 1925–28, and editor of the journal Communist International from 1935.