We figure the Civil War was judgment because of slavery, which was the main issue it was fought over.
Once again this is the conclusion you come to when you worship an inerrant Bible instead of Jesus. Jesus, (and Paul for that matter), certainly would have opposed slavery in the US but He would have been just as opposed to the civil war.
Jesus lived in a country that virtually enslaved His fellow Jews and what did He do about it. He said love your enemy, turn the other cheek etc. His message was that the way you encounter injustice including slavery that we are to use God's message of peace and love.
It isn't always that clear about what the answer is to be but that is where we are called by Jesus to start. For example when we look at this war on ISIS how do we apply that message? Dropping bombs on people is not loving the ISIS fighters but maybe it is loving to those who are being persecuted by them. All we can do is to do our best to prayerfully work out a loving solution.
The Civil War was only God's judgement in the sense that it is the natural consequence of turning away from the commandment to love our neighbours as ourselves. Maybe the north should have purchased slaves and freed them. Maybe they should have provided safe places for slaves to escape to. Maybe they should have evangelized with the teachings of Jesus in the south where in many cases they espoused Christianity but them misused it in the way that fundamentalists or inerrantists do today.
Just maybe the Civil War was just another power grab with slavery being just another side issue.
He has told you, O man, what is good ; And what does the LORD require of you But to do justice, to love kindness, And to walk humbly with your God.