The Civil War was the deadliest war in American history. Altogether, over 600,000 died in the conflict, more than World War I and World War II combined. A soldier was 13 times more likely to die in the Civil War than in the Vietnam war.

One reason why the Civil War was so lethal was the introduction of improved weaponry. Cone-shaped bullets replaced musket balls, and beginning in 1862, smooth-bore muskets were replaced with rifles with grooved barrels, which imparted spin on a bullet and allowed a soldier to hit a target a quarter of a mile away. The new weapons had appeared so suddenly that commanders did not immediately realize that they needed to compensate for the increased range and accuracy of rifles.

The Civil War was the first war in which soldiers used repeating rifles (which could fire several shots without reloading), breechloading arms (which were loaded from behind the barrel instead of through the muzzle), and automated weapons like the Gatling gun. The Civil War also marked the first use by Americans of shrapnel, booby traps, and land mines.

Outdated strategy also contributed to the high number of casualties. Massive frontal assaults and massed formations resulted in large numbers of deaths. In addition, far larger numbers of soldiers were involved in battles than in the past. In the Mexican War, no more than 15,000 soldiers opposed each other in a single battle, but some Civil War battles involved as many as 100,000 soldiers.