I don't understand why some people think a civil war absolutely must be preceded by states' intent of secession. It's far more common to foment an uprising against the current government and try to instate a new one. If distinctive territories even could be drawn in a second American civil war, it would more likely be along county lines, and in some cases even along civic borders. Indeed, the most recent "battle maps" of civil warfare depict which cities with how many people are controlled by which faction, with the geographic territory not being attributed to anyone. (See this map of Syria for a current example.)
Anyway, as polarized as we are at the moment, I don't think another civil war is likely to happen anytime soon. The only scenario where I can see this coming anywhere close to happening is if the electoral vote gets split in a 269-to-269 tie, and the odds of that are astronomical in scale. And even then, it would take the government itself splitting apart over the issue, taking sides against each other, and rallying their respective supporters behind them.
The military would be an 'X' factor: they might stand firmly on the side of the appointed president because they see it as their duty to do so, or a number of them could decide it's their duty as the country's protectors to get the appointed out of power. This is why I always shake my head at people who assume the army will crush any uprising before it starts; there's a good chance some of the combatants will be soldiers and officers themselves, and it's not unheard of for people to change loyalties mid-conflict.