There are many historians who consider James Buchanan as one of the worst presidents of the White House. That’s mainly because he took no action to try to unify a country that was sharply divided over slavery, and did nothing to stop the Southern states from seceding in the lead-up to the Civil War.
While his pre-presidency resume was quite impressive, he blew it during his presidential years. He allowed the residents of each territory, and not the U.S. government, to decide whether or not they should allow slavery. Every step and every action he did as a president made the country grow even more divided.