All this constituted what is known as the "Seven Days' Battles." It was a brilliant operation, attended at every step by heroic fighting on both sides, and by consummate skill on both--for if Lee's successful operation for his enemy's dislodgment was good strategy, McClellan's successful withdrawal of his army from its imperilled position to one in which it could not be assailed, was scarcely less so. But still more dramatic events were to follow. McClellan had been driven away from the immediate neighbourhood of the Confederate capital, but his new position at Harrison's Landing was one from which he might at any moment advance again either upon Richmond or upon Petersburg, which was afterward proved to be the military key to the capital. His army was still numerically stronger than Lee's, and it might be reinforced at any time, and to any desired extent, while Lee had already under his command every man that could be spared from other points. More important still, the fighting strength of McClellan's forces had been bettered by the battling they had done. The men were inured to war work now, and had improved in steadiness and discipline under the tutelage of experience. Except that its confidence in its general was somewhat impaired, the Army of the Potomac was a stronger and more trustworthy war implement than it had been at the beginning. So long as it should remain where it was, Lee must keep the greater part of his own force in the intrenchments in front of Richmond, and the seat of war must remain discouragingly near the Confederate capital. In the meanwhile a new Federal force, called the Army of Virginia, had been sent out from Washington under General John Pope, to assail Richmond from the north and west, while securely covering Washington. Pope's base was at Manassas, and his army had been pushed forward to the line of the Rappahannock, where there was no army to meet it and check its advance upon Richmond. Lee must act quickly. For should Pope come within striking-distance of Richmond on the northwest, McClellan's army would very certainly advance from the east, and Richmond would be threatened by a stronger force than ever before. But Lee could not move in adequate force to meet and check Pope's advance, without leaving Richmond undefended against any advance that McClellan might see fit to make. His perplexing problem was to compel the withdrawal of McClellan, and the transfer of his army to Washington.