The military have the tanks, the air support, the logistics, the surveilence net, the miscelaneous support equipment, and all the training to use everything.
A split within the military, that gets real ugly real fast.
And then, because there were demonstrably some absolute sadists demonstrably present in the armed forces during my lifetime (e.g. https://en.wikipedia.org/wiki/The_Hooded_Man), perhaps the conscientious objectors will be convinced to fight anyway, to stop the sadists.
It also matters what such a civil war be about — Is it between those who would seize power and those who would prevent it? Is it the same borders as the old Civil War? Is it city-vs-rural?
If there is one (still an if), and if it is Trump vs. the constitution… it's still not impossible for such a conflict to be without a single shot fired. Conversely, if it's between two groups of cities neither of which will consent to the other's choices for president, it could have every major city in the US reduced to radioactive debris.
