this seems to be a great piece. Thanks!

My understanding is that not long before the war, the north had pushed to tax import (that the north supplied to the south) at an extremely high rate (well above 50% import tax).

The result was that the south was paying for 80% of the federal gov. (since there was no income tax).

the south said, nope, we are leaving. and the north realized the would destroy the tax base for the gov, and so did not let them leave.

Is this an accurate summary?

(As you say, by the end of the war is was about slavery, but in the beginning Lincoln himself said he did not play to change any laws about slavery, so it seems it was not about that.)