Short answers, pulled from the story.
The Battle of Antietam produced 22,727 total casualties on both sides in a single day, making it the bloodiest day in American history. The Union suffered 12,410 casualties including 2,108 dead, while the Confederates had 10,316 casualties with 1,547 dead. This represented 25% of the Federal force and 31% of Lee's Army of Northern Virginia.
The Battle of Antietam was fought on the 17th of September 1862, near Sharpsburg, Maryland, along Antietam Creek. It was part of Lee's Maryland Campaign, the first field army-level engagement in the Eastern Theater of the Civil War to take place on Union soil.
The Union's strategic victory at Antietam gave President Lincoln the political confidence to issue the preliminary Emancipation Proclamation on the 22nd of September 1862, five days after the battle. Secretary of State William H. Seward had previously advised Lincoln to wait for a significant Union victory so the proclamation would not appear to be issued out of desperation.
After Antietam, Lincoln issued the Emancipation Proclamation linking Union victory to the end of slavery. Both Britain and France had already abolished slavery, and neither government could afford to be seen supporting the Confederacy once the war was framed as a fight over slavery. The political cost of opposing the United States outweighed any benefit of recognizing the Confederate government.
Special Order 191 was Lee's detailed battle plan discovered by two Union soldiers, Corporal Barton W. Mitchell and First Sergeant John M. Bloss of the 27th Indiana Volunteer Infantry, wrapped around three cigars. It revealed that Lee had divided his army across multiple locations including Harpers Ferry and Hagerstown. McClellan waited roughly eighteen hours before acting on this intelligence, squandering a major opportunity to defeat Lee's divided forces.
President Lincoln relieved McClellan of command of the Army of the Potomac on the 5th of November 1862, citing his overcaution and his failure to pursue Lee's retreating army across the Potomac. Despite repeated orders from Lincoln and the War Department, McClellan declined to advance from September 17 through October 26. He was replaced by General Burnside on November 9, effectively ending his military career.