Complex sentence: Born into a Prussian family with a long military tradition, Rundstedt entered the Prussian Army in 1892 and rose through the ranks until World War I, in which he served mainly as a staff officer.
Split into simpler sentences: Born into a Prussian family with a long military tradition, Rundstedt entered the Prussian Army in 1892. During World War I, he served mainly as a staff officer.