In the autumn of 1914, he joined Polish Military Organisation, helping to create its structures in the areas of Lublin and Warsaw.

In 1917, Kasprzycki created the Association of Polish Military Organizations (Związek Polskich Organizacji Wojskowych).

On 1 January 1929 Kasprzycki was promoted to the rank of Brigadier general, and in July 1931, he became Deputy Minister of Military Affairs.

On 12 May 1935, after the death of Józef Piłsudski, Kasprzycki was named Minister of Military Affairs by the President of Poland and remained in this post until 30 September 1939.

In December 1937, he was named chairman of Polish Association of Research Expeditions, and due to his efforts, a road along the ridge of Gubałówka was built.

In the spring of 1939, Kasprzycki's wife Maria née Strychalska committed suicide, and his son, who at that time was a high school student, broke all relations with the father.