Mikhail Gorbachev has died

The term "Civil War" affirms the United States' (i.e. Northern) viewpoint that the secessions of the southern states were illegitimate and the conflict that ensued was between citizens of the same country.

If you think that the secession of southern states was a legitimate act and the formation of the CSA was legitimate, you can't call it a civil war. It's a war between two sovereign states, of which the United States (i.e. "The North") would be seen as the aggressor, hence "War of Northern Aggression."

The motivation behind seceding was predominantly maintaining slavery, though then a deeper question might be, when is secession legitimate? And is there any justification that the United States might simply accept secession of a state from the Union?

> though then a deeper question might be, when is secession legitimate?

It's an entirely subjective question. Never, according to the state being seceded from or revolted against, and always, according to the secessionists and revolutionaries.

It's worth mentioning that the same American government that added the Second Amendment and spoke in florid prose about the blood-sacrifice of patriots and rebellion against governments also put down rebellions against itself.

>And is there any justification that the United States might simply accept secession of a state from the Union?

No. Cultural reasons aside, there is simply too much money and infrastructure at stake (to say nothing of political instability threatening its superpower status) for the US to be willing to lose even a single state.