The war - and 600,000 deaths - could have been avoided entirely if the north had put together a workable plan to pass a constitutional amendment to eliminate slavery peacefully. By that I mean if their goal was to eliminate slavery. Peacefully.
They tried - the south would listen.
There were 15 slave states. In order to pass a constitutional amendment in the face of their united opposition there would have had to be 46 states voting in favor. Do the math.