Complex sentence: He served for 12 years and four terms until his death in 1945, the only president ever to do so, and a central figure in world events during the mid- 20th century, leading the United States during a time of worldwide economic depression and total war.
Split into simpler sentences: He served for 12 years and four terms, and was the only president ever to serve more than eight years. He was a central figure in world events during the mid- 20th century, leading the United States during a time of worldwide economic depression and total war.