That's the difference between a revolution and civil war. Around 750,000 men died fighting the Civil War; that's roughly 55% of all American deaths in every war we've fought, combined. Our deaths in WWII barely broke half that.
The U.S. Civil War was incredibly bloody. It's said, quite accurately, that all U.S. history before the war led up to it, and all after the war flowed from it. That was over a period of around four years, between only two actors, and discounts civilian deaths and so forth. For reference, the Syrian civil war, which is at the forefront of peoples' minds as an atrocity, has lasted for longer than that and has killed a combined total of 300-470k people, military and civilian alike, from a dozen or more actors, using modern weaponry.
Know what's even worse? Of the dead soldiers in the U.S. Civil War, only ~ a third died in combat. Two thirds of those deaths were down to disease, infection, starvation, mistreatment in prison camps, &c. It's a scar on the national psyche which was allowed to fester when the ex-Confederate Right dismantled Reconstruction and nobody stopped them. There aren't enough words to describe how profoundly the Civil War and its aftermath affected this country.