I have finished Paul Johnson's wonderful history of The United States and I thought I'd give an answer to:  "Why did we fight a civil war?"  It's not all that simple a question because the two sides were fighting about different things.  

I read over 1,200 pages about Mao and Lenin in the past few weeks.  I distill this into two lessons:  1) Never allow hardship to lead to the concentration of political power  2) Beware of politicians whose hearts apparently bleed for the poor.