December 1919. From October 10, 1919, he served as an operations officer of the Polish 1st Legions Infantry Division, composed mostly of his colleagues from the Polish Legions. Next, he served as Chief of the 3rd Operational Group Branch led by General Edward Rydz-Śmigły. Next, Bortnowski served as Chief of the 3rd Branch of the 3rd Army, and from October 1920, as chief of staff of the 3rd Army led by Zygmunt Zieliński. Interwar period After the conclusion of the Polish–Soviet War, Bortnowski traveled to Paris, France, where he began training at the École Supérieure de Guerre on November 1, 1920. In September 1922, he graduated and returned to Poland where he received further training at various posts, notably in the staff of the Army Inspectorate in Wilno (modern Vilnius, Lithuania). On August 15, 1924, he was promoted to the rank of colonel and in October of the following year, he became the commanding officer of the Kutno-based 37th Infantry Regiment. After the May Coup d'État in 1926, he served as the Chief of the 3rd Branch of the General Army Staff for two months starting in November. In February 1928, he was appointed commander of the 26th Infantry Division in Skierniewice. From June 1930, he was transferred to Poznań where he was appointed commander of the 14th Infantry Division. In October 1930, was assigned to the Army Inspectorate in Toruń as an officer. On November 1, 1931, he took command of the Zamość-based 3rd Legions Infantry Division, one of the most prestigious units of the Polish Army. On January 1, 1932, Bortnowski was promoted to the rank of brigadier general by President Ignacy Mościcki. Starting from October 12, 1935, he served as an Inspector General of the Armed Forces at Toruń. In the Autumn of 1938, Bortnowski took command of the Independent Operational Group Silesia which took participation in the occupation of Czechoslovak territory resulting from the Munich Agreement. Specifically, Bortnowski's troops occupied the Czechoslovak territory of Zaolzie. The popularity of Bortnowski after his successful invasion and occupation of Zaolzie was so great back in Poland, that he was planned to replace Marshal Edward Rydz-Śmigły as commander-in-chief when the latter was to run for the presidency in the planned 1940 elections. Prior to the outbreak of the September Campaign in September 1939, Bortnowski was promoted to the rank of division general on March 1, 1939, and became the commanding