What does it take to start a civil war?

Perhaps the most famous civil war in history began on April 12, 1861, when secessionist Southerners attacked the United States Army garrison at Fort Sumter, South Carolina. Subsequent hostilities embroiled the American states in bloody battles that slaughtered a large portion of their youthful male population. About a quarter of each army was killed and nearly half a million wounded, many of them grievously.

Even after the South surrendered, in 1865, the aftermath of the war saw intense resentment that, it is said, continued even until after the last veteran of that war finally died of old age.