"No single battle decided the outcome of the Civil War. Several turning points brought reversals of an apparently inexorable momentum toward victory by one side and then the other during the war. Two such pivotal moments occurred in the year that preceded Antietam. Union naval and military victories in the early months of 1862 blunted previous Southern triumphs and brought the Confederacy almost to its knees. But Southern counteroffensives in the summer turned the war around. When the Army of Northern Virginia crossed the Potomac River into Maryland in September, 1862, the Confederacy appeared to be on the brink of victory. Antietam shattered that momentum. Never again did Southern armies come so close to conquering a peace for an independent Confederacy as they did in September 1862. Even though the war continued and the Confederacy again approached success on later occasions, Antietam was arguably, as Karl Marx and Walter Taylor believed, the event of the war."
Historian James McPherson Crossroads of Freedom: Antietam (2002)