Raszewski was born 29 February 1864 in a small village called Jasień near Poznań (Czempiń Municipal), in the Province of Posen in the Kingdom of Prussia.

Kazimierz was the second son of a szlachta (Polish landed gentry) Ignacy Raszewski and Józefa Koczorowska.

[2] The commander of the Regiment in Prudnik was lieutenant Rosenberg, a friend of Raszewski's father with whom he served in the Husaren-Regiment Nr.

After many years, in his autobiography, he stated that he wouldn't become an officer without the support he received from Rosenberg while in Prudnik.

[1] After graduating from military school in Nysa in 1887, he came back to Prudnik as a second lieutenant and became the commander of the platoon.