It was at this moment that Grant began creeping cautiously away toward the rear door of Richmond. To keep a vigilant enemy in entire ignorance of such a tremendous move was, of course, impossible, but the system and discipline which he had instilled into his army almost accomplished the feat. Indeed, so rapidly and silently did the troops move, so perfect were the arrangements for transporting their baggage and supplies, so completely were the details of the whole undertaking ordered and systematized, that over a hundred thousand men, infantry, cavalry, and artillery, with their horses, hospital and wagon trains, and all the paraphernalia of a vast army virtually faded away, and when Lee gazed from his intrenchments on June 13, 1864, there was no sign of his opponent and he did not discover where he had gone for fully four days. In the meantime, Grant had thrown his entire army across the James River and was advancing, horse and foot, on Petersburg, the key to the approach to Richmond from the south, and Butler, whose troops had been extricated from their difficulties, was ordered to seize it. Petersburg was at that moment wholly unprepared to resist a strong attack. Indeed, there were only a handful of men guarding the fortification, the capture of which would case the fall of Richmond, but Butler was not the man to take advantage of this great opportunity. On the contrary, he delayed his advance and otherwise displayed such wretched judgment that the Confederates had time to rush reënforcements to the rescue, and when Grant arrived on the scene the intrenchments were strongly occupied. Notwithstanding this the Union commander ordered a vigorous assault, and for three days the troops were hurled against the breastworks without result. The last attack was made on June 18, 1864, but by this time 10,000 Union soldiers had been sacrificed and Lee had arrived in person with strong support. Grant accordingly, abandoning his efforts to carry the place by storm, began to close in upon it for a grimly sullen siege.