McClellan at last reached the neighbourhood of Richmond, and placed his great army on the eastern and northern fronts of the Confederate capital. But still permitting his imagination to mislead him, he confidently believed the Confederate forces to be quite twice as numerous as they were in fact. So instead of pressing them vigorously, as a more enterprising and less excessively cautious commander would have done, he proceeded to fortify and for weeks kept his splendid army idle in a pestilential swamp, whose miasms were far deadlier than bullets and shells could have been. At the end of May the Confederates assailed his left wing, believing that a flood in the river had isolated it from the rest of the army, and a bloody five days' battle ensued, with no decisive results, except to demonstrate the fighting quality of the troops under McClellan's command. Still he hesitated and fortified, and urgently called for reinforcements. These to the number of forty thousand were on their way to join him, marching directly southward from Washington. But the Confederates had been more fortunate than their foes. They had found their great commander, a piece of good fortune which did not happen to the Federal armies until nearly two years later. After the battle of Seven Pines at the end of May and the beginning of June, Robert E. Lee assumed personal command of the forces defending Richmond, and from that hour the great game of war was played by him with a sagacity and a boldness that had not been seen before. Lee's problem was to drive McClellan's army away from Richmond, and transfer the scene of active hostilities to some more distant point. To that end he must prevent the coming of McDowell with his army to McClellan's assistance. Accordingly he ordered Jackson to sweep down the Shenandoah valley, threatening an advance upon Washington in its rear, thus putting the Federals there upon their defence. He rightly believed that the excessive concern felt at the North for the safety of the capital would make Jackson's operations an occasion of great alarm. The result was precisely what Lee had intended. Jackson swept like a hurricane through the valley, moving so rapidly and appearing so suddenly at unexpected and widely separated points as to seem both ubiquitous and irresistible. The Federal army which was marching to reinforce McClellan was promptly turned aside and sent over the mountains to meet and check Jackson. While it was hurrying westward, Jackson suddenly slipped out of the valley and carried his "foot cavalry"--as his rapidly marching corps had come to be called--to the neighbourhood of Richmond, where Lee was ready to fall upon his adversary in full force, striking his right flank like a thunderbolt, pushing into his rear, pressing him back in successive encounters, threatening his base of supplies on the York River, and finally compelling him to retreat to the cover of his gunboats at Harrison's Landing on the James.