The American Civil War
Abraham Lincoln won the presidency in 1860 opposing slavery's expansion into new territories, and eleven southern states seceded before he took office, forming the Confederacy. Their own declarations of secession named the protection of slavery as the cause. Four years of fighting killed around 750,000 people — more Americans than every other American war combined — as rifled muskets that could kill at 300 yards met generals still marching men shoulder to shoulder. Lincoln's Emancipation Proclamation in 1863 turned a war for the union into a war against slavery; the Union's industrial capacity and manpower ground the South down, and Lee surrendered at Appomattox in 1865. Slavery ended and the union was settled as permanent — but within a generation the racial order it had underwritten was substantially rebuilt through law and violence.
A belief system rises, spreads, institutionalizes, and then schisms — changing the rules people accept as legitimate.