It would not be interesting to give the details of the many manoeuvres by each commander, but before long Lee saw he could not hold his position at South Mountain, and he retreated toward Sharpsburg, near the stream of water known as Antietam Creek. He was thus on the flank of any Federal force that might attempt to save Harper's Ferry. Naturally he held the fords of the Potomac, so that in case of defeat the way to Virginia was open. [Illustration: GENERAL LEE'S INVASION OF THE NORTH. The Confederate army under General Lee twice invaded the north. The first invasion was brought to a disastrous end by the Battle of Antietam, September 17, 1862. The second invasion ended with greater disaster at Gettysburg, July 1-3, 1863. Gettysburg was the greatest and Antietam the bloodiest battles of the war.] Still Lee and Jackson were separated by a wide stretch of mountain, river, and plain, and McClellan was aware of the fact. He had the opportunity to cut off each division in detail, but lacked the nerve and dash to do it. There were subordinates in the Army of the Potomac who yearned for just such a chance, but McClellan's timidity and excessive caution deprived him of another golden opportunity, as it had done before and was soon to do again. The position of Lee was among a range of hills, which, following the form of a crescent, extended from the lower point of Antietam Creek to a bend in the Potomac. Jackson was straining every nerve to join Lee, but his men were taxed beyond endurance, and many of them fell by the roadside from utter exhaustion, only a portion reaching Sharpsburg on the 16th. The full Confederate army did not exceed 40,000, while McClellan, who arrived on the opposite side of Antietam Creek, that afternoon, had 70,000. Instead of attacking at once, he waited two days, and thus gave Lee time to gather many thousand stragglers. [Illustration: ANTIETAM BRIDGE.] BATTLE OF ANTIETAM OR SHARPSBURG. Finally, when McClellan had no excuse for further delay, and the enemy was in fine form, he opened the attack on the morning of the 17th. To reach Lee the Union commander had to cross the creek, which was spanned by three bridges, each defended by Confederate batteries.