Without a doubt if Union broke we would have had wars between Northern and Southern states. Without a doubt America wouldn't have become a power, as it is today.
That doesn't excuse Lincoln ignoring the Constitution nor does it excuse the rise of the all powerful Federal Government.
As to WWII, WWII was direct result of WWI not being finished... it basically was WWI part 2. The main reason why WWII happened was as a result of our involvement in WWI, which broke the stalemate.
While we do not have a ways to be check any of the "What Ifs", it is very doubtful that Hitler would have rose to power without the Versile treaty and the whole culture that arose due to the "Stab in the back" believes in Germany.
So as far as we are concern