Abolitionists vs. Slave-owners, States Rights vs. Federal Government, the causes were many. For several years, politicians had been walking a fine line trying to pacify the Southern slave-owners and those who opposed slavery.
Issues came to a head with the election of Abraham Lincoln as president in 1860. Eleven Southern states would secede from the Union [AL, SC, NC, VA, GA, FL, TX, MS, LA, AR, TN]. Maryland, Kentucky, Missouri and Delaware remained in the Union, but permitted slavery. Virginia would split in 1863, with the formation of West Virginia as a Union state.
The North was industrialized and had the advantage of population. The South was agrarian and the majority of its population were slaves.
Fort Sumter, SC was fired upon by Southern forces in April 1861, igniting hostilities. The war was expected to be a short one, but lasted four years.
Early battles were won by the Confederacy. The Confederate commanders were far more successful than the Union military leaders. The tide would turn at Gettysburg, PA, where Gen. Robert E. Lee was defeated by Maj. Gen. George Meade.
A string of ineffective Union generals was eventually replaced by Gen. Ulysses S. Grant, who would lead the Union forces to victory. Lee surrendered to Grant at Appomattox Court House on 9 April 1865. Five days later, President Lincoln was assassinated by John Wilkes Booth. Any chance the Confederacy had for a smooth transition in returning to the Union was lost.
For far more detail see: https://en.wikipedia.org/wiki/American_Civil_War
Family connections: Brothers Sylvester, John and Aaron Crail served with the Union. Sylvester enlisted in the 79th Indiana Infantry and later served in the Veterans Reserve Corps. John and Aaron were with the 124th Indiana Infantry. During the torrential rains at the Battle of Kennesaw Mountain in 1864, Aaron became sick and was sent to the military hospital. He would be sent home and would die from "lung fever" in 1868. He left a widow and 6 children. Aaron's brothers would suffer war related health problems the rest of their lives.