The admission of new states to the Union exacerbated tensions over the issue of slavery. As new western territories sought statehood, debates emerged over whether they would permit or prohibit slavery. The Compromise of 1850 and Kansas-Nebraska Act attempted to address this by appealing to popular sovereignty but failed to resolve conflicts. John Brown's raid against pro-slavery settlers in Kansas heightened national tensions. Abraham Lincoln's election in 1860 as the first Republican president led southern states to secede, forming the Confederate States of America in an effort to protect the institution of slavery. The outbreak of war became inevitable as the North and South took opposing stances on whether states had the right to secede, with the attack on Fort Sumter