Stroop enlisted in the Prussian Army in 1914 and served in several infantry regiments along the Western Front. He was wounded in action near La Bassée in October 1914.[5] To the outrage of his devoutly Catholic mother, Stroop married Katharina B., the daughter of a minister from the Protestant Church of Lippe, on 3 July 1923.[6] Katharina Stroop remained a loyal and obedient wife despite her husband's many infidelities.[7] Their marriage produced a daughter, Renate Stroop, who was born in February, 1928.[6] Their son, Olaf Stroop, was born in February 1936.[8]