The leaders of the country who were elected president during the Secession Era, and helped to make United States History, were Millard Fillmore, Franklin Pierce and James Buchanan.

President Millard Fillmore and the Secession Era: Millard Fillmore Presidency (1850-1853)
Important events in United States History during the presidency of Millard Fillmore includes the Compromise of 1850, aka the Omnibus Bill, Uncle Tom's Cabin and Commodore Perry's Mission to Japan - United States History for Kids:

President Franklin Pierce and the Secession Era: Franklin Pierce Presidency (1853-1857)
Important events in United States History during the presidency of Franklin Pierce includes the Gadsden Purchase, The Kansas-Nebraska Act and the Ostend Manifesto - United States History for Kids:

President James Buchanan and the Secession Era: James Buchanan Presidency (1857-1861)
Important events in United States History during the presidency of James Buchanan includes the Panic of 1857, The Dred Scott Decision, Harpers Ferry, The Pony Express. The events surrounding Secession are detailed in the Civil War website.

Crash Course in American History: Summaries of Events in the Secession Era
A fast overview, or crash course in American History, during the Secession Era can be obtained from the interesting and important articles on United States History for Kids that can be found in the section. There are summaries and definitions of the key historical events for each of the presidencies of the Secession Era from 1850 - 1860.