Our 14th president of the United States was Franklin Pierce. He was one of our younger presidents, being only 48 when he assumed office. He's known primarily for fighting in the Mexican-American War pre-presidency and signing the Kansas-Nebraska act during his one term.
Pierce, a democrat, was a "dark horse" for the nomination, being fairly unknown within the party before rising to popularity. However, he entered the office distraught, since two months prior, his eleven year old son was killed in a train crash.
In office, Pierce's attempts at expansion raised criticism from Northerners. They believed Pierce was trying to spread slavery. His expansion attempts included his negotiations with the UK and Spain over the Central American coast and Cuba.
But what really caused controversy during his presidency was the Kansas-Nebraska Act. With the help of Stephen A. Douglas, Pierce made plans to build a trans-continental railroad to Western territories. It was deemed these Western territories would be able to decide the question of slavery for themselves. Northerners and Southerners both rushed to Kansas in an attempt to spread their influence for either non-slavery or slavery states. The conflict from the power struggle culminated in Bleeding Kansas- shootings erupted, and are now pointed to as a prelude to the Civil War.
Kansas returned to a peaceful condition by the end of Pierce's term, but the democrats decided to nominate James Buchanan over Pierce, as Buchanan was less controversial. Though following the compromises between the North and the South under Millard Fillmore, with everyone expecting a relatively peaceful era for Pierce to preside over, the storm began brewing anew during Pierce's expansion of the nation.
Franklin Pierce is consistently rated by historians in the worst quartile of United States presidents, though not to as extreme a degree as his successor, Buchanan. Indeed, what's particularly interesting about US leadership in this era is how Abraham Lincoln's 3 predecessors, Taylor, Fillmore, and Pierce, all rank in the lowest quartile while Lincoln himself is consistently one of the top 3 presidents in the ranking.
Sources: Wikipedia, Biography.com, White House.gov, History1900s.about.com