Up to September 14, 1862, the Confederates had felt safe on the western side of a ridge of mountains running from north to south through Maryland. J. E. B. Stuart’s cavalry provided a much-desired screen, keeping the Federals out. But on September 14, McClellan’s forces began to attack through Turner’s Gap—which separates two sections of South Mountain—and Crampton’s Gap, which separates South Mountain from Maryland Heights. Parts of the area still look a good deal as they did in 1862, and even the most casual tourist remarks on both the beauty and the difficulty of the terrain.

The battle for the two mountain passes raged all day, with reinforcements for both sides arriving at various times. The battle was touch-and-go, but the superior federal numbers eventually held, and both gaps were in Union hands by nightfall. The Confederate leaders still did not realize the extent to which McClellan understood their dispositions, and there was some wonder expressed at their headquarters. Why had McClellan suddenly become so aggressive? Lee did not believe the situation was lost, however. He had new sets of orders out that evening, with the plan for all sections of the Army of Northern Virginia to converge on the little town of Sharpsburg.