“But why did they go to war to “free the slaves?” That could have been done peacefully using the constitutional amendment process in - say, 1848 - or even earlier.”
For some people the end of slavery was just a bellwether for the increasing and disproportional power of the Northern states. Also, a Constitutional amendment to end slavery prior to the Civil War would have never mustered the necessary state legislature votes for ratification.
The other issue wasn’t just North vs. South but a clash of Industrial/Urban culture with Agrarian/Rural culture. A paradigm that we see occurring today with urban leftists clashing with rural Americans.
If it is true that Lincoln did not have the votes to peacefully amend the pro-slavery U.S. Constitution prior to the war - there is evidence that was the case - then perhaps Lincoln's plan after the House Divided speech was to use war to overthrow the pro-slavery U.S. Constitution. Many people are firmly convinced Lincoln "fought to free the slaves."
But to levy war against the states Lincoln would need a pretext. He possibly found that when he used the navy to create the Gulf of Tonkin Incident.
I meant to say the Fort Sumter Incident.