What is the bloodiest event in United States history, in terms of casualties?

The story behind the answer

Battle of Antietam is correct because it produced about 23,000 casualties (killed, wounded, or missing) on a single day—September 17, 1862—the bloodiest single day in American history. That far exceeds the U.S. tolls of Pearl Harbor (~2,400 dead), September 11 (~3,000 dead), and U.S. casualties on D‑Day (~10,000).

Fought in Maryland between Robert E. Lee’s Army of Northern Virginia and George B. McClellan’s Army of the Potomac, Antietam halted the Confederate invasion and gave Abraham Lincoln the opening to issue the Preliminary Emancipation Proclamation, reshaping the war’s purpose and discouraging European support for the Confederacy. Memory tip: Antietam’s “A” can stand for “All in one day”—the Civil War’s fiercest single-day bloodshed.