Complex sentence: At the outbreak of World War I in 1914 he joined the German army despite being over 40 years old and served on both the western and eastern fronts, rising to the rank of lieutenant in the artillery.
Split into simpler sentences: At the outbreak of World War I in 1914 he joined the German army despite being over 40 years old. He served on both the western and eastern fronts, rising to the rank of lieutenant in the artillery.