The two armies that faced off in the Battle of Antietam were the Army of the Potomac and the Army of Northern Virginia.

The Army of the Potomac, commanded by Union General George McClellan, was tasked with stopping the Confederate invasion of the North led by General Robert E. Lee and his Army of Northern Virginia. This confrontation on September 17, 1862, marked a crucial moment in the Civil War, as it was not only one of the bloodiest battles in American history but also the first significant engagement on Northern soil, highlighting the war's escalating intensity.