The key phrase that tore the country apart in 1860 is part of Amendment V in the Constitution: “…nor shall any person …be deprived of life, liberty, or property without due process of law” The Supreme Court ruled in the Dred Scott decision that not to allow slaves in Northern "free" states would deprive a slave owner of his property if he decided to move there.
An earlier section of the Dred Scott decision in the Supreme Court found, based on the situation when the Constitution was approved in 1787, that a slave or the descendant of a slave was not a citizen of the United States and could not sue in court.
By the 1850s, many people in Northern “free” states opposed slavery but the balance of slave and free states in the Senate made change in the status quo difficult. However, the ruling that slaves could be taken into any free state upset all compromises. Leaders in slave states rejoiced in their victory and those in “free” states determined to prevent it.
As the presidential election of 1860 approached, the Democratic Party was dominated by representatives of Southern states who shouted down Stephen Douglas’s plan for popular sovereignty or a vote on slavery and insisted that the Supreme Court ruling was the final word. Slavery was allowed everywhere! The Democratic Party fractured into three conventions. Two of them nominated Southerners who ran on implementing the Dred Scott decision.
Southern states did not send representatives to the Republican Convention which nominated Abraham Lincoln. The Republican platform proposed to retain free states and limit slavery to the states where it was established. In other words, not to allow implementation of the Dred Scott decision.
This was unacceptable to the South and shortly after Lincoln’s victory was clear, South Carolina broke their ties with the United States. Other Southern states followed. A new government, called the Confederate States of America, was organized in February 1861.
When Lincoln was inaugurated on March 4, he stated his intention to avoid war. That changed with the Confederate attack on Fort Sumter.