From 1915, Bierut was active in the cooperative movement. In 1916, he became trade manager of the Lublin Food Cooperative, and from 1918 was its top leader, declaring the cooperative's "class-socialist" character. During World War I, he stayed at times at Hempel's apartment in Warsaw and took trade and cooperative courses at the Warsaw School of Economics. In Warsaw, he established contacts with Maria Koszutska and in December 1918 some form of association with the newly created Communist Workers' Party of Poland (KPRP), from which, according to his later testimony, he withdrew in fall 1919. Bierut kept assuming ever higher offices in the cooperative movement. In 1919 he and Hempel went to Prague, where they represented the Polish cooperatives at the congress of their Czechoslovak counterparts. Bierut's increasingly radical views, however, eventually hindered his cooperative career and caused his departure from the leadership of the movement, beginning in 1921. From 1921, he officially functioned as a member of the KPRP.