It was be nice and neat with each side having different uniforms from the opposing states fighting in rural battlefields.
It will be more like the Spanish Civil War with no clearly defined front lines with combat ants wearing uniforms and lots of urban warfare where many civilians are killed, starved and raped.
Lots of “private justice” by militias, gangs and individuals.
All the comforts of modern life we take for granted such as reliable electricity, gas, food, medical and potable water will be viewed as luxuries.
The dead will line the roadways and old diseases that affect 3rd world countries will reappear as millions die.
The end of the United States.
Sad to agree, a Second Civil war would be a modern horror show. To be avoided at all cost. In many ways for like the French Revolution than the little dust up in 1861.