It was Erlon's failure to be present _either_ with Ney _or_ with Grouchy, either upon the left or upon the right, either at Quatre Bras or at Ligny, while each of those two actions were in doubt, which made it possible for Wellington's troops to stand undefeated in the west, for the Prussians to retire--not intact, but still an army--from the east, and for both to unite upon the day after the morrow, the Sunday, and destroy the French army at Waterloo. It is upon Erlon's blunder or misfortune that the whole issue turns, and upon the Friday, the 16th of June, in the empty fields between Quatre-Bras and Ligny, much more than upon the famous Sunday at Waterloo, that the fate of Napoleon's army was decided. In order to make this clear, let us first follow what happened in the operations of Napoleon's right wing against the Prussians opposed to it,--operations which bear in history the name of "the Battle of Ligny." LIGNY "_If they fight here they will be damnably mauled._" (Wellington's words on seeing the defensive positions chosen by the Prussians at Ligny.) Napoleon imagined that when he had crossed the Sambre with the bulk of his force, the suddenness of his attack (for, though retarded as we have seen, and though leaving troops upon the wrong bank of the river, it was sudden) would find the Prussian forces in the original positions wherein he knew them to have lain before he marched. He did not think that they would yet have had the time, still less the intention, to concentrate. Those original positions the map upon p. 41 makes plain. The 124,000 men and more, which lay under the supreme command of Blucher, had been spread before the attack began along the whole extended line from Liège to Charleroi, and had been disposed regularly from left to right in four corps d'armée. The first of these had its headquarters in Charleroi itself, its furthest outpost was but five miles east of the town, its three brigades had Charleroi for their centre; its reserve cavalry was at Sombreffe, its reserve artillery at Gembloux. The Second Corps had its headquarters twenty miles away east, at Namur, and occupied posts in the country as far off as Hannut (thirty miles away from Charleroi). The Third Corps had its headquarters at Ciney in the Ardennes, and was scattered in various posts throughout that forest, its furthest cantonment being no nearer than Dinant, which, by the only good road available, was nearer forty than thirty miles from Napoleon's point of attack. Finally, the Fourth Corps was as far away as Liège (nearer fifty than forty miles by road from the last cantonment of the First Corps), and having its various units scattered round the neighbourhood of that town.