How did the Civil War alter the course of American History so severely? Exactly what did it change?

Just what about the Civil War was such a strong dividing line in American History?

The Civil War was one of the most destructive large-scale conflicts in the Western World between the Napoleonic Wars and the onset of World War 1, with more American lives lost than in any other American conflict combined.

But in addition to the bloodshed and violence, it was the first event since the Revolution that altered the political, social, and economic threads that held together the stitches of America. (Editors of U.S. History.com, 2020)

What questions about the path America was heading did the Civil War answer?

The Civil War resolved two big questions that the Revolution did not:

Whether the U.S. was to be a confederacy of individual states or one indivisible country controlled my a sovereign national government.
Whether the U.S., a country built around ideals of freedom and equality, would continue on as one of the largest slave holding countries in the world.

(McPherson, 2011)