Complex sentence: Franklin Pierce (November 23,1804October 8,1869) was the 14th President of the United States (1853--1857), whose inability to calm tensions over slavery kept the country on the path to the American Civil War.
Split into simpler sentences: Franklin Pierce (November 23,1804October 8,1869) was the 14th President of the United States (1853--1857). His inability as president to calm tensions over slavery kept the country on the path to the American Civil War.