In one sense the war was unnecessary. Research has confirmed what I have never doubted: a fraction of the material cost of the war would have been enough to buy the freedom of every slave in the United States. Lincoln proposed exactly this, but it seems that almost nobody else was interested, North or South.
War was inevitable. If the South would have been allowed to leave the Union, there would have been ongoing border wars, innumerable disputes, grievances over commerce and trade. The longer the war was put off the more would have died when the war to end the issue finally took place. Lincoln was the Greatest President this nation ever had, followed by Washington, and then Reagan.