Why Was the Battle of Antietam the Deadliest Day in American History?

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Sept. 17 2012 4:19 PM

The Deadliest Battle

Why did so many people die at Antietam?

Antietam Photos

Today is the 150th anniversary of the Battle of Antietam, the bloodiest battle in American history. About 23,000 men died, were wounded, or went missing over the course of the single-day battle. Why was Antietam so deadly?

L.V. Anderson L.V. Anderson

L.V. Anderson is a Slate assistant editor. She edits Slate's food and drink sections and writes Brow Beat's recipe column, You're Doing It Wrong. 

Advanced technology, unwise tactics, and terrible decision-making. The combination of new rifles that could be shot with great accuracy from far away and old-fashioned battle lines led to unprecedented deaths in the Battle of Antietam (and in the Civil War in general). As in other Civil War battles, both sides in Antietam arranged their infantry shoulder-to-shoulder in two long parallel lines before marching into battle. This type of linear formation made sense in earlier years, when military weaponry consisted mostly of smoothbore muskets (which were accurate only at short range) and bayonets (which, likewise, could only be used at close range). But by the beginning of the Civil War, rifling—the use of helical grooves in the barrel of a weapon, which stabilize a bullet, leading to greater shooting accuracy—was widespread. Now soldiers could make an aimed shot from 100 yards away and shoot into an enemy line with hope of hitting someone from 400 yards away. Armed with rifled muskets, a defensive line could do serious damage when attackers attempted to charge.

dead on battlefield
A Federal soldier is buried, while a Confederate lays unburied.

Alexander Gardner/Library of Congress.

Better explosives also contributed to Antietam’s unprecedented bloodiness. On both sides, infantry units were bolstered by artillery batteries consisting of three to six cannons, which could be loaded with canisters. A canister was a tin can filled with about 120 bullets, which meant that firing a canister was similar in effect to firing a machine gun.

These technological advances were present in other Civil War battles, too, and it’s thanks to chance and some really bad decision-making that Antietam earned the superlative of most deadly. The Union forces outnumbered the Confederate forces by about 2 to 1, but the Union general, George B. McClellan, didn’t wield the Union forces to his advantage. Instead of sending his strongest units to flank the Confederates, he sent weaker units, which weren’t quite up to the task. Worse, McClellan was stationed too far away from these units to be able to know about their failures and send reinforcements, which made this strategy fairly useless.

At the same time, Union Gen. Ambrose Burnside foolishly sent troops across a narrow bridge that led to a steep hill occupied by Confederate riflemen. Even though the Confederate riflemen were severely outnumbered, they had such an advantageous position that they were able to forestall the Union forces for hours. (Adding to the stupidity of Burnside’s decision to cross the bridge was the fact that there was an easily crossed ford that he could have traversed instead—he just hadn’t found out where it was before the battle began.)

Meanwhile, the Confederate forces had the advantages of more experience, better command, and a high proportion of artillery batteries to infantrymen. The fact that the Union side vastly outnumbered the Confederates—and yet was wildly incompetent by comparison—meant that McClellan’s forces were able to absorb huge losses while simultaneously inflicting huge losses on the Confederates, leading to the deadliest one-day battle in American history.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Richard Slotkin, author of The Long Road to Antietam: How the Civil War Became a Revolution.

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