Complex sentence: Berthold Guthmann, born in 1893, volunteered for military service in the Imperial German Army after the outbreak of the First World War, as did his two brothers, with one of them killed in action at Verdun.
Split into simpler sentences: Berthold Guthmann, born in 1893, volunteered for military service in the Imperial German Army after the outbreak of the First World War. Guthmann's two brothers also volunteered for military service with one of them killed in action at Verdun.