Lee still lay intrenched on the hills behind Fredericksburg, and there Hooker ordered General Sedgwick to hold him with part of the army while he himself, with another and more powerful part, crossed the Rappahannock River by a ford twenty-seven miles above. By this move he hoped to get behind Lee and then crush him, as nut-crackers would crush a nut, by closing in on him with a front and rear attack. This was not a strikingly original plan. It was in fact merely a flanking movement on a huge scale, but compared to Burnside's performance it was highly scientific and the vast superiority of the Union forces almost insured its success. Hooker was certainly convinced that he had at last solved the great problem of the war and that Lee was practically in his power. Indeed, as his flanking army forded the river, he issued an address of congratulation in which he informed his troops that they had the Confederates in a position from which they must either "ingloriously fly" or come out in the open where certain defeat awaited them. But "Fighting Joe" was soon to learn the folly of crowing until one is out of the woods, for as he emerged from the forests sheltering the fords, he discovered that Lee's army had not remained tamely in its intrenchments, but had quietly slipped away and planted itself squarely across his path. For a moment the Union commander was fairly astounded. He had prophesied that his adversary would fly from Fredericksburg, but he had not expected him to move so soon or in this direction. Indeed, his well-matured plans were based on the supposition that Lee would remain where he wanted him to be until he was ready to spring his trap, quite forgetting that though it is easy to catch birds after you have put salt on their tails, it is rather difficult to make them wait while you salt them. As a matter of fact, Lee had taken alarm the moment his cavalry scouts reported his opponent's movement towards the fords and, realizing that he would be caught if he remained where he was, he had rapidly departed from Fredericksburg, leaving only enough force to occupy Sedgwick's attention. Even then he was in a precarious position, for Hooker's flanking army alone outnumbered him and the force threatening Fredericksburg would certainly start in pursuit of him as soon as it discovered that the bulk of his army had withdrawn from that city. All this was equally clear to Hooker after his first gasp of astonishment, and as he hurriedly ordered Sedgwick to attack Fredericksburg with part of his forces and to send the rest as reënforcement against Lee, he confidently believed that his foe had delivered himself into his hands.