The series of events that eventually led to the outbreak of Civil War were complex, but were all centered on the main issue of slavery. As the US expanded west and more states were added to the Union, there was an effort to keep the balance of power between slave and free states, but there were also strong forces leading the abolition movement as well as pro-slavery factions. After decades of attempted appeasement, the states of the South finally seceded, thus sparking the Civil War.