The lesson to learn from the civil war is rhetoric. 20 years of rhetoric created the situation when young teenagers of both the North and South were anxious to kill the other. This same situation has existed in the USA from the 1960's until today when colleges  and high schools have been overwhelmed by teachers all too ready to demonize the USA, and lionize Communism and Socialism.

And now, as back then in 1861, this can not end peacefully.