Even allowing for some pro-Confederate respondents, I find the number of voters who chose the Civil War--20 out of 30--surprisingly low. For those who didn't choose it, I'd be interested to know what your reasons are.
In my case it is because I consider the right of people to national self-determination important. The Southern republics had political interests divergent from those of their counterparts in the North. Refusing to accept the legitimacy of succession was not worth throwing away tens of thousands of lives over and - although under different circumstances I'd have stubbornly fancied war against the CSA on humanitarian grounds - at the time I suspect said country posed too great a threat to the Union to be prudently tangled with in a full-blown conflict. For now I think a diplomatic, non-violent resolution and an agreement for peaceable coexistence would have been preferable.
>implying that Southerners are a separate nationality
'Divergent political interests' in this case means slavery. How in the world does a socialist defend the right of slaveowners to try and destroy the United States (because they lost an election) because they want to continue owning, oppressing, and crippling other human beings?
A better solution at the time would have been to negotiate with southerners their states rights for abolition of slavery, and once the preemptively didn't work, apply a traditional neoliberal policy and blockade the south from trade which would have made them reconsider their position after a couple years of impoverishment.