We’re at a point where the breakup of the US and the breakup of the EU would not be an end of civilization. It would give pockets of civilization that still exist the ability to save themselves.
The MSM is extremely squeemish when it comes to casualties. They will scream bloody murder if more than 1000 US soldiers are killed in a conflict.
If another civil war happened, the potential casualties would be horrific—especially given modern weapons. However, if a state or region tried to secede from the US, my $$ says they would be mum about the resulting casualties.
It would be an open question to see how many bodies would pile up before public opinion said “let them go”. Likely, the Federal Government would try a blockade or crippling sanctions to minimize the casualties. Plus, cut off all benefits to people in those areas.