On 24 April 1920, Jędrzejewicz, now a captain, signed a military convention with Ukraine's "Ataman" Semen Petlura which paved the way for the Polish Army's 1920 Kiev Expedition. Next he served as Section II chief successively to Generals Kazimierz Sosnkowski and Gustaw Zygadłowicz. In September–November 1920, as a major, he was the Polish Army's liaison officer to allied Belarusian forces.