How the US civil war started, in a nutshell:

North: "Slavery is immoral. We should plan to get rid of slaves."

South: "Are you fucking kidding me? Our whole economy is based on slavery! We'd better leave before that happens."

North: "What? No you can't leave! Because then we couldn't tax you anymore!"

So the North moved their military

And the South said "They are about to strike! Let's strike first!"

and that is how it started.