But when he sent Jackson back and detached other strong forces to hold the avenues of escape from Harper's Ferry, his army was badly scattered, its several parts lying at too great a distance from each other for ready coöperation. During the consequent days of waiting, McClellan was advancing in leisurely fashion to meet the Confederate movement, and his army was every day adding to its strength by the hurrying forward of fresh regiments and brigades to its reinforcement. Finally Lee issued an order setting forth in detail his plan for concentrating his scattered forces. Copies of this order, showing the exact location of each part of the army and the movements to be made by each, were sent to all of the corps commanders. One of those copies was lost, and fell into McClellan's hands. For once that most leisurely of generals was in a hurry. His opportunity had come to destroy the Army of Northern Virginia by beating it in detail. He threw a strong force forward to assail certain of its positions. The assault proved successful, but the success did not come so quickly as it should have done. By determined fighting Lee gained time in which to bring his scattered forces together again at Sharpsburg before his adversary could fall upon him in force. There, on Antietam Creek, on the 17th of September, 1862, was fought a battle which is reckoned the bloodiest of all the war, in proportion to the numbers engaged. McClellan had seventy thousand men in line, Lee forty thousand. The struggle began early in the morning and continued until after nightfall. The fighting on both sides was as heroic and as determined as any that was ever done in the world. At the end of it all both sides claimed the victory, and neither had in fact won it. Neither had been able to drive the other from his position. Neither had broken the other's lines or gained any decisive advantage. And when morning came again neither side was willing to renew the contest, and neither would retire from the field. For a whole day the two armies lay facing each other in grim defiance, each ready to receive the other should it attack, but neither venturing to make the assault. After twenty-four hours of defiant waiting, Lee slowly retired to the Potomac, while McClellan lay still, not venturing to follow his adversary. Lee crossed unmolested into Virginia and took up a position within easy striking distance, but his adversary made no attempt to strike. McClellan presently advanced and stretched his great army along the Potomac. But he assumed an attitude of defence, calling insistently for reinforcements, though his army outnumbered Lee's about two to one. He had succeeded in checking Lee's invasion of the North and turning it back. He was content with that, and in spite of President Lincoln's urgency he refused to do more, till at last General Burnside was ordered to assume command in his stead.