Answer: well i personally would choose the civil war there are MANY things each army could have done in those 6 years i believe to prevent all that bloodshed. such as Union General Ulysses S. Grant makes what he later recognizes to be his greatest mistake by ordering a frontal assault on entrenched Confederates at Cold Harbor, Virginia. The result was some 7,000 Union casualties in less than an hour of fighting.
Explanation: the civil war is mostly known for the mistakes both of the generals did on both sides of the union and confederacy if some mistakes could have been prevented the death toll could have dropped from 750,000 deaths to maybe 200,000