11. CHAPTER V.--A flank march in presence of the enemy is a most difficult and dangerous operation. In the case of Blucher’s movement, there was little actual danger from Grouchy, as events proved, but in face of a vigorous enemy the Prussians would have been in a perilous position. It was possible for an active enemy to seize the bridges over the Dyle at Moustier and Ottignies, and fall upon Blucher’s flank. The latter was not exposing his communications, for his real communications were with Liege; he had temporarily abandoned them when he marched on Wavre; but if attacked during his march his position would not have been by any means safe. If defeated, whither would he have fallen back? This is the chief danger of a flank march: the lack of a good, or even of any, line of retreat. As a rule, a flank march, being away from the general line of advance or retreat, has necessarily to be made on lesser roads, and the difficulty of ample movement from one to another, or of rapid deployment or change of front, becomes prodigious. Blucher, if attacked during this march, would most probably have left one corps to detain the enemy, while he, with the other three corps, resumed his march towards Wellington; for to turn back would have been as dangerous as to advance. But if his way had been barred he would have fallen back on Brussels rather than upon Louvain, as he would still have a chance of joining Wellington. If Blucher had been so attacked and defeated, Grouchy would have been able to deal a terrible, in all probability a crushing, blow on Wellington’s left flank. It is interesting, but not particularly profitable, to speculate as to what course events would have taken had Grouchy been up in time to prevent Blucher’s flank march, and had checked him. Would Wellington have fallen back on Brussels with Blucher, and fought again under the city walls against Napoleon and Grouchy combined? In that case, the weight of numbers would have been very much in favour of the allies, and the great object of Napoleon’s plan of campaign--to prevent the junction of the two armies--would have been thwarted. If Blucher, after being checked, had fallen back on Louvain, while Wellington was still engaged with Napoleon, it seems obvious that Grouchy’s extra numbers thrown into the fight would have caused the Duke’s overthrow, for it would not then have been necessary for Napoleon to detach against the Prussians; Wellington was too seriously engaged to be able to withdraw, and the defeat would have been complete. But after all, such speculation as this might be continued indefinitely; and every campaign might be discussed and argued to a hundred different conclusions by re-modelling the conditions or improvising situations. A campaign, like a chess problem, admits of more than one solution.