SHARPSBURG, Md. -- The National Park Service is kicking off four days of events marking the 150th anniversary of the Civil War Battle of Antietam (ann-TEE'-tem) near Sharpsburg, Md.

Stonewall Jackson biographer James Robertson Jr. heads the list of speakers at Friday's opening ceremonies.

The lectures and battlefield hikes culminate Monday with a commemoration on the anniversary date.

The battle along Antietam Creek on Sept. 17, 1862, was the bloodiest day of combat on U.S. soil. More than 23,000 Union and Confederate soldiers were reported killed, wounded or missing during 12 hours of fighting with no clear winner.

Gen. Robert E. Lee withdrew his troops to Virginia the next day. The Confederate retreat gave President Abraham Lincoln the political strength to issue a preliminary Emancipation Proclamation five days after the battle.

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  • Battle of Antietam

    This 1862 photograph made available by the Library of Congress shows casualties from the Battle of Antietam near the church of the pacifist Dunker sect near Sharpsburg, Md. When dawn broke along Antietam Creek on Sept. 17, 1862, cannon volleys launched a Civil War battle that would leave 23,000 casualties on the single bloodiest day in U.S. history and mark a crucial pivot point in the war. And yet it might never have occurred - if not for what a historian calls a "freakish" twist of fate. Days earlier, a copy of Gen. Robert E. Lee's detailed invasion orders, wrapped around a few cigars, accidentally fell in a farm field and were discovered by Union infantrymen who passed their stunning find up the chain of command, spurring action. (AP Photo/Library of Congress)

  • Battle of Antietam

    Sarah Spall, 5, looks over a stone wall at Burnside Bridge at the Antietam battlefield July 4, 2005, where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, September 17,1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    Tourists visit the Antietam battlefield July 4, 2005, where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17, 1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    In this Dec. 4, 2010, file photo, luminaries are seen across Antietam National Battlefield in Sharpsburg, Md., to commemorate the soldiers who were killed or wounded during the three-day Civil War battle.

  • Battle of Antietam

    A cannon sits in a cornfield at the Antietam battlefield July 4, 2005, where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17, 1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    In this 1862 photo made available by the Library of Congress, President Abraham Lincoln and Gen. George B. McClellan sit in the general's tent after the Battle of Antietam near Sharpsburg, Md. McClellan's skill in organizing and preparing troops was what made Lincoln elevate him to command, even though the president had long been frustrated by another defining trait of "Little Mac" - his paralyzing deliberateness and tendency to grossly exaggerate the forces he faced. As a general, he was the temperamental opposite of Gen. Robert E. Lee. (AP Photo/Library of Congress, Alexander Gardner)

  • Battle of Antietam

    This Dec. 3, 2011, photo provided by Dave Maher shows some of the 23,110 luminaries placed throughout the northern portion of the Antietam battlefield to commemmorate each of the casualties near Sharpsburg, Md. The battle marked a crucial pivot point in the war. And yet it might never have occurred - if not for what a historian calls a "freakish" twist of fate. Days earlier, a copy of Gen. Robert E. Lee's detailed invasion orders, wrapped around a few cigars, accidentally fell in a farm field and were discovered by Union infantrymen who passed their stunning find up the chain of command, spurring action. (AP Photo/David Maher)

  • Battle of Antietam

    This July 21, 2012, photo shows a cannon in front of the "Best" farm outside of Frederick, Md. where Gen. Robert E. Lee drafted orders detailing his plans for the September 1862 Confederate invasion of Maryland. A misplaced copy of the document was found near this site by a Union soldier and passed up the chain of command, helping to set the stage for the pivotal Battle of Antietam on Sept. 17, 1862. (AP Photo/Chris Sullivan)

  • Battle of Antietam

    This 1864 photo made available by the Library of Congress shows Confederate Army Gen. Robert E. Lee. When dawn broke along Antietam Creek on Sept. 17, 1862, cannon volleys launched a Civil War battle that would leave 23,000 casualties on the single bloodiest day in U.S. history and mark a crucial pivot point in the war. And yet it might never have occurred - if not for what a historian calls a "freakish" twist of fate. Days earlier, a copy of Lee's detailed invasion orders, wrapped around a few cigars, accidentally fell in a farm field and were discovered by Union infantrymen who passed their stunning find up the chain of command, spurring action. (AP Photo/Library of Congress, Julian Vannerson)

  • Battle of Antietam

    This 1862 photo made available by the Library of Congress shows soldiers next to a lone grave after the Battle of Antietam near Sharpsburg, Md. When dawn broke along Antietam Creek on Sept. 17, 1862, cannon volleys launched a Civil War battle that would leave 23,000 casualties on the single bloodiest day in U.S. history and mark a crucial pivot point in the war. And yet it might never have occurred - if not for what a historian calls a "freakish" twist of fate. Days earlier, a copy of Gen. Robert E. Lee's detailed invasion orders, wrapped around a few cigars, accidentally fell in a farm field and were discovered by Union infantrymen who passed their stunning find up the chain of command, spurring action. (AP Photo/Library of Congress, Alexander Gardner)

  • Battle of Antietam

    In this Saturday, June 30, 2012, photo, Dave Maher, wearing a uniform like those of Union army soldiers and carrying a replica Civil War rifle, stands in front of the simple white church building of the pacifist Dunker sect on the Antietam battlefield near Shaprsburg, Md. The church was repaired after being badly damaged in the fighting on Sept. 17, 1862. Historical photographs from the war "hooked" Maher on the Civil War as a kid. The volunteer battlefield guide said, "Before that, people had only seen those heroic portraits of men in battle," referring to engravings that illustrated popular journals. But here, "people weren't just seeing a dead body. This was somebody's brother. And these were contemporaries - it wasn't 150 years ago then." (AP Photo/Chris Sullivan)

  • Battle of Antietam

    This 1862 photo made available by the Library of Congress shows dead Confederate soldiers in a ditch on the after the Battle of Antietam near Sharpsburg, Md. When dawn broke along Antietam Creek on Sept. 17, 1862, cannon volleys launched a Civil War battle that would leave 23,000 casualties on the single bloodiest day in U.S. history and mark a crucial pivot point in the war. And yet it might never have occurred - if not for what a historian calls a "freakish" twist of fate. Days earlier, a copy of Gen. Robert E. Lee's detailed invasion orders, wrapped around a few cigars, accidentally fell in a farm field and were discovered by Union infantrymen who passed their stunning find up the chain of command, spurring action. (AP Photo/Library of Congress, Alexander Gardner)

  • Battle of Antietam

    A view of Antietam battlefield July 4, 2005 where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17, 1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    Tracks cut through a field at the Antietam battlefield July 4, 2005, where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17, 1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    A cannon sits in a cornfield at the Antietam battlefield July 4, 2005, where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17,1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    Vehicles drive through the Antietam battlefield July 4, 2005 where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17, 1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    Tourists look at markers setup during a visit to the Antietam battlefield July 4, 2005 where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Maryland. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17,1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)

  • Battle of Antietam

    A cannon sits in a cornfield at the Antietam battlefield July 4, 2005 where General Robert E. Lee ended his first invasion of the North in September 1862 in Antietam, Marlyand. The battle claimed more than 23,000 men killed, wounded, and missing in one single day, Sept. 17, 1862, and led to Lincoln's issuance of the Emancipation Proclamation. (Photo by Joe Raedle/Getty Images)