Lincoln warned the South in his Inaugural Address: "In your hands, my dissatisfied fellow countrymen, and not in mine, is the momentous issue of civil war. The government will not assail you.... You have no oath registered in Heaven to destroy the government, while I shall have the most solemn one to preserve, protect and defend it."

Lincoln took his oath to "preserve, protect and defend" the government of the United States of America to heart and called for the enlistment of over 75,000 able bodied men to re-unite the country.

All war is terrible but the Civil War with Americans fighting each other had to be one of the worst wars ever.  Over 600,000 were killed during the war and the South's economy was mostly obliterated.  The war lasted through April 1865 and only days after the surrender Lincoln was fatally shot at Ford's theater.