He attacked those Prussians with the bulk of his forces; and although he was outnumbered even upon that field, he defeated the Prussians at Ligny. But the defeat was not complete. The Prussians were free to retire northward, and so ultimately to rejoin Wellington. They took that opportunity, and from the moment they had taken it Napoleon was doomed. We have further seen that Grouchy, who had been sent after the Prussian retreat, might, if he had seen all the possibilities of that retreat, and had seen them in time, have stepped in between the Prussians and Wellington, and have prevented the appearance of the former upon the field of Waterloo. Had Grouchy done so, Waterloo would not have been the crushing defeat it was for Napoleon. It would very probably have been a tactical success for Napoleon. But, on the other hand, we have no ground for thinking that it would have been a final and determining success for the Emperor. For if Wellington had not known quite early in the action that he could count upon the arrival of the Prussians, he would not have accepted battle. If, as a fact, he had found the Prussians intercepted, he could have broken contact and retreated before it was too late. Had he done so, it would simply have meant that he would later have effected a junction with his allies, and that in the long-run Napoleon would still have had to fight an allied army immensely superior to his own. All this is as much as to say once more what has been insisted upon throughout these pages; Waterloo was lost, not upon Sunday, June 18th, but two days before, when the 63,000 of Napoleon broke and drove back the 80,000 of Blucher but failed to contain them, failed to drive them eastward, away from Wellington, or to cause a general surrender, and failed because the First French Army Corps, under Erlon, a matter of 20,000 men, failed to come up in flank at the critical moment. We have seen what the effect of that failure was; we have discussed its causes, and we must repeat the main fact for military history of all those four days: the breakdown of Napoleon's last desperate venture turned upon Erlon's useless marching and countermarching between Quatre Bras and Ligny, two days before the final action of Waterloo was fought. This being so, the battle of Waterloo must resolve itself into two main phases: the first, the beginning of the struggle with Wellington before the Prussians come up; the second, the main and decisive part of the action, in which both Prussians and English are combined against the French army.