Complex sentence: His wife Louise was married briefly during the Great War 15 years earlier in 1918, to a German named Frederick Bosner, a young man who worked for the U.S. State Department but was actually a spy for Germany.
Split into simpler sentences: His wife Louise was married briefly during the Great War 15 years earlier in 1918, to a Frederick Bosner. He was a young man who worked for the U.S. State Department but was actually a spy for Germany.