IMO, reconstruction is the unfinished part.

The war was the war. It ended a long time ago. Reconstruction failed and we are still dealing with the fallout today.

When I was in high school in the mid-90s history ended with WWII. I didn’t learn about Vietnam until I took a second level history course in college.

To me, Tulsa doesn’t fit into a general history course. It falls into a course on the failures of reconstruction and the civil rights movement (I view the civil rights movement as a natural response to the failures of reconstruction).

Almost every major historical event is complex in its origins. Slavery existed from the birth of our nation. If the issue was so contentious, why did the 13 original colonies agree to form a union? Why didn’t we partition into free and slave states then? Why did it take 70 years for the issue to bubble up and cause war? Why did we engage in war, when other countries emancipated their slaves peacefully? Why did the Union take up arms: To free the slaves or to preserve the union?

A response of “slavery” is too simplistic in my mind. And, ironically, it goes to your point that we fail to learn about context and history at a deeper level. Instead, the answer is just slavery or you are a member of the lost cause or a southern apologist.