On August 16, 1920, during the Battle of Warsaw, he was assigned as the commanding officer of Polish 4th Cavalry Brigade, which later became the core of the so-called "Nieniewski's Operational Group" during the Battle of the Niemen River. After the end of hostilities of the Polish–Soviet War, Nieniewski graduated from a Generals' Course and on January 10, 1921, he was promoted to Colonel of Cavalry.