Coincidently with the news that those distant advancing thousands were his own men and would turn this doubtful struggle into a decisive victory for the Emperor came the news--unexplained, inexplicable--that Erlon's troops would advance no further! That huge distant body of men, isolated in the empty fields to the westward; that reinforcement upon which the fate of Napoleon and of the French army hung, drew no nearer. Watched from such a distance, they might seem for a short time to be only halted. Soon it was apparent that they were actually retiring. They passed back again, retracing their steps beyond the western horizon, and were lost to the great struggle against the Prussians. Why this amazing countermarch, with all its catastrophic consequences was made will be discussed later. It is sufficient to note that it rendered impossible that decisive victory which Napoleon had held for a moment within his grasp. His resource under such a disappointment singularly illustrates the nature of his mind. Already the Emperor had determined, before any sign of advancing aid had appeared, that if he were left alone to complete the decision, if he was not to be allowed by fate to surround and destroy the Prussian force, he might at least drive it from the field with heavy loss, and, as far as possible, demoralised. In the long struggle of the afternoon he had meant but to press the Prussian line, while awaiting forces that should complete its envelopment; these forces being now denied him, he determined to change his plan, to use his reserves, the Guard, and to drive the best fighting material he had, like a spearhead, at the centre of the Prussian positions. Since he could not capture, he would try and break. As the hope of aid from Erlon's First Corps gradually disappeared, he decided upon this course. It was insufficient. He could not hope by it to destroy his enemy wholly. But he could drive him from the field and perhaps demoralise him, or so weaken him with loss as to leave him crippled. Just at the time when Napoleon had determined thus to strike at the centre of the Prussian fine, Blucher, full of his recent successes upon his right and the partial recapture of the village of St Amand, had withdrawn troops from that centre to pursue his advantage. It was the wrong moment. While Blucher was thus off with the bulk of his men towards St Amand, the Old Guard, with the heavy cavalry of the Guard, and Milhaud's cavalry as well--all Napoleon's reserve--drew up opposite Ligny village for a final assault. Nearly all the guns of the Guard and all those of the Fourth Corps crashed against the village to prepare the assault, and at this crisis of the battle, as though to emphasise its character, a heavy thunderstorm broke over the combatants, and at that late hour (it was near seven) darkened the evening sky. It was to the noise and downpour of that storm that the assault was delivered, the Prussian centre forced, and Ligny taken.