I’ll put it forefront that I argue the Civil War was caused by the polarization of the country, sectionalism. The Wilmot Proviso from the Mexican-American War is seen by many as the “first round” to the political conflict. Another major event was The Compromise of 1850, which admitted CA as a free state as decided by popular sovereignty, established a Fugitive Slave Law, and forbade slave trade in D.C. However, this fugitive slave law was often resisted by antislave and abolitionist Northerners, ultimately driving a wedge between the North and the South. Lincoln also addressed the writer of Uncle Tom’s Cabin, Harriet Beecher Stowe, as “…the little woman who wrote the book that made this great war,” as this look made tons of northerners abolitionists; I recommend looking into it. Another major event was the Kansas-Nebraska Act leading to “Bleeding Kansas.” Also John Brown’s Raid at Harpers Ferry causing southerners see the raid as proof of the north’s intentions to use slave revolts to destroy the south. There’s a lot of events but I hope this gives you a starting point.