1. Slavery, Not Abstraction, Was the Cause
Decades of historiography have settled the central question of causation more firmly than popular memory often acknowledges. Slavery, and specifically the determination of slaveholding states to protect and extend it, drove secession. The Confederate states said so openly in their secession documents, their constitution, and the public statements of their leaders, who described the subordination of Black people as the cornerstone of their new government. Arguments that emphasize tariffs, states' rights, or cultural difference are not wrong that those tensions existed, but they collapse under scrutiny when separated from slavery: every significant political crisis of the antebellum decades, from the Missouri controversy to Bleeding Kansas to the Compromise of 1850, was a crisis about the future of enslaved labor.
Why it matters: Getting the cause right matters enormously for understanding what the war's outcome meant and why Reconstruction was contested so bitterly. A war over slavery ending in emancipation is a very different story from a war over vague constitutional principles.