American Civil War: George B. McClellan's Army of the Potomac halts the first invasion of the North by Robert E. Lee and his Army of Northern Virginia in the single-day Battle of Antietam, the bloodiest day in American military history.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union to preserve slavery in the United States, which they saw as threatened because of the election of Abraham Lincoln and the growing abolitionist movement in the North. The war ended with Union victory, the dissolution of the Confederacy and the abolition of slavery, freeing four million African Americans.