Complex sentence: Harden later told Hirschfeld that the affair was the greatest political mistake of his life, like many later observers, attributing the affair as the root cause of World War I and the fall of the Second Reich, inevitable without Eulenburg's moderating influence.
Split into simpler sentences: Harden later told Hirschfeld that the affair was the greatest political mistake of his life. Like many later observers, attributing the affair as the root cause of World War I and the fall of the Second Reich, inevitable without Eulenburg's moderating influence.