A Pennsylvania-born Democrat, deeply devout in his faith and the only bachelor elected to the presidency, Buchanan rejected slavery as an indefensible evil but, like the majority of his party, refused to challenge the constitutionally established order.
Even before he became president, he supported the various compromises that made it possible for slavery to spread into the western territories acquired by the Lousiana Purchase and the Mexican War. (Particularly hurtful to the cause of restraining slavery's spread was the Kansas-Nebraska Act of 1854, for example, allowed settlers to determine the status of slavery in their proposed state constitutions.)
In his inaugural address, the 15th president tacitly encouraged the Supreme Court's forthcoming Dred Scott decision, which ruled that Congress had no power to keep slavery out of the territories.
More damaging to his name, though, was his weak acquiescence before the secessionist tide—an unwillingness to challenge those states that declared their intention to withdraw from the Union after Lincoln's election. Sitting on his hands as the situation spiraled out of control, Buchanan believed that the Constitution gave him no power to act against would-be seceders.
To his dying day, he felt that history would treat him favorably for having performed his constitutional duty. He was wrong.