I think both sides were stupid if you ask me. The South was idiotic for ever expecting to win when the Union army outnumbered them something like 1 to 5 I think (although admittedly, many Union soldiers in places like California had little to no training) and they could never expect to live up to the Union's manufacturing capabilities. Plus, firing on Fort Sumter sent off a horrible message for the South. It says that you don't peacefully want to secede, but that you are using force to do so, and thus, the Union is going to respond back with force.
The Union is stupid for going along with a war that ended up killing 600,000 people and absolutely jeopardizing the South, that led to more people suffering, and then having to waste millions of dollars to repair it when the war was over. If it were up to me, I'd just take a Marshall Plan approach to the issue of slavery, giving economic aid and benefits to States that join the Union and abolish slavery, while refusing to trade with Confederate states, and allow any escaped slaves that make it to a Union state to keep their freedom. There are better ways to resolve your problems without bloodshed.
Anyhow, the Civil War is responsible for the estrangement that Yankee and Dixie America has with each other. Ironically, instead of uniting us, it only divided us more. There is always going to be that "us/them" mentality now because of the war.