Monday marks 150 years since the bloodiest day in U.S. history, the Civil War Battle of Antietam in Maryland, which left almost 23,000 Union and Confederate soldiers dead, wounded, missing or captured.

While Union forces suffered a heavier casualty toll - 12, 400 Union to 10, 300 Confederate casualties - and military historians consider the battle a draw, President Abraham Lincoln called it a Union victory and said it showed that the Union army could enforce orders coming out of Washington. Five days later, Lincoln issued the Preliminary Emancipation Proclamation. When it went into effect on January 1, 1863, it freed slaves in the rebellious Confederate states and made the abolition of slavery an official U.S. policy. Read the original Preliminary Emancipation Proclamation from the National Archives here.

"Antietam enabled Lincoln to identify the nation's cause with the cause of liberty for men and women everywhere and at all times, and had it not occurred, it is quite possible that America never would have become the beacon of freedom the world now recognizes," The Baltimore Sun writes in an editorial Monday.

The Battle of Antietam was brutal and up close for the 131,000 troops engaged, 87,000 on the Union side and 45,000 for the Confederacy. In the part of the battlefield known as the Sunken Road, so much blood was spilled that dirt turned to mud, so much so that the road was later given the name Bloody Lane.

The horrific fighting and thousands of dead littering the battlefield also led to some of America's first photographs showing the carnage of war. The images, taken by Alexander Gardner, an assistant of famed Civil War photographer Matthew Brady, were exhibited at Brady's New York City studio. In its Lightbox blog, TIME.com shows those images and writes about how Americans reacted to them in 1863.

To see how much or how little the battlefield has changed in 150 years, check out NPR's "Then and Now" photo project. The network sent photographer Todd Harrington to the battlefield, where he used a Civil War-era camera to take images from the same spots that Gardner shot from shortly after the battle.

Re-enactments commemorated the battle over the weekend, including the fighting, as described in The Washington Post, and the retreat of the thousands of wounded, as reported by the Journal-News of Martinsburg, West Virginia.

USA Today's Chuck Raasch reports that the lessons of Antietam extend to the battlefields of today and even into the emergency medical treatment we often take for granted.

"Every time you see an ambulance run down the road as a result of a 911 call, that is the Battle of Antietam going down the road in front of you," Raasch quotes George Wunderlich, executive director of the National Museum of Civil War Medicine, as saying.

Monday, the National Park Service will honor the dead from Antietam with a reading of all their names in a ceremony at Antietam National Cemetery. The 3 p.m. event will include the names of those buried in the national cemetery and three nearby Confederate cemeteries. The Park Service is also asking for help identifying casualties of the battle who may be buried elsewhere.

The National Endowment for the Humanities on Monday will live-stream an Emancipation Proclamation event, as Civil War historians and scholars assume they're living in 1962 and discuss the national scene Lincoln faced as he issued the Preliminary Proclamation. The event will take place at the Smithsonian Museum of American History beginning at 1:30 p.m. ET.

Antietam history from the National Park Service

Learn more about the Battle of Antietam from the Civil War Trust.