Ok leaving nukes out of this, a second American Civil War would be absolutely horrifying because
A) We have more advanced weapons now
And
B) Our people live such soft cushy lives they would be completely unprepared for the consequences of war fought on our own soil.
Most of the people in the south back in the day knew how to farm, hunt, and fish so they were able to pull through and survive the aftermath of Sherman's March to the Sea.
Today pretty much all Americans are spoiled by mass produce foods sold in supermarkets and served in restaurants along with clean running water, vehicles to drive us anywhere we want, and other modern conviences like electricity, healthcare, and internet.
If one side tried to block the other side from having access to this stuff in order to starve them into defeat like the Union did the south so many more people would die and would be unable to rebuild their lives even after the war ended. We are no longer the same people we used to be 150 years ago, we are spoiled by modern conviences. So only your hardcore survivalists would have a chance of pulling through.
Plus unlike in the old days the American government today has proven ineffective in actually rebuilding countries after defeating them in war. Look at Iraq as the perfect example. So the side thar loses would be in a huge mess that they would most likely never recover from.
Seriously, Trump should be punished for even entertaining the idea of plunging America into another civil war over this impeachment crap!