I trace the most important, and to me the most decisive, “what if?” situation about Waterloo in not the exact day of Waterloo but just two days before: 16th of June 1815, when the battles of "Quatre-Bras" and "Ligny" were fought simultaneously. When speaking of Waterloo, these three days of actions from the 16th to 18th should be considered as a whole and as subparts of the general term of "Waterloo", in a more broad sense. This is because of the fact that after Napoleon had crossed the border and had engaged the Allies, the events and decisions leading to the French disaster happened rapidly and continuously as a 3-days long battle (similar to Leipzig maybe), though not happened on a continuous front and though with a “no-action” day of 17th. Thus, Waterloo cannot be abstracted from the battles of the 16th. In 16th, when Marshal Ney was fighting against Wellington at Quatre-Bras, General D'Erlon's I Corps of French Army was supposed to be under Ney's command and Ney taken into account this corps to launch his decisive assault on Wellington to master the crossroads at Quatre-Bras and also to beat Wellington's army. After outnumbered Wellington managed to equate the balance of manpower at Quatre-Bras, Ney had to wait for the arrival of the second infantry corps under his command, namely D'Erlon's I corps. At the same time and some miles east, Napoleon, however, was fighting the Prussians at Ligny and was thinking about using this same corps. Although Napoleon crushed the Prussians, he was not able to put them in a total rout and his plan was to use D'Erlon's corps to envelop the right wing of the Prussian army. Clearly this would cause to a total disaster for the Prussians and most probably they would not be able to even think to continue fighting. Due to the lack of effective communications and some other effects, General D'Erlon, whose corps was marching towards Quatre-Bras in a line just between Napoleon at Ligny and Ney at Quatre-Bras, had received inconsistent orders from both of his superiors, Ney and Napoleon, saying him to march towards both west and east, respectively. He had to change the marching direction of his corps twice and waste so many hours resulting in an unfortunate situation that he could intervene decisively NONE of the engagements. He passed the 16th by just marching back and forth between the two important battlefields. As a result, Ney couldn’t beat English Army but only could prevent them joining the Prussians at Ligny during 16th and then he was driven back to Frasnes, and Napoleon couldn’t totally enjoy his victory at Ligny by eliminating the Prussian Army. Both Allied armies managed to survive the day and continue to their course of rejoining afterwards. The orders given to D'Erlon in 16th both by written and oral were very controversial and were not totally put under light. What if this discrepancy did not happen? What if Napoleon or someone from his staff did not ask D'Erlon to march Ligny by by-passing his direct superior, Ney and by neither considering Ney's plans nor informing him? What if Ney did not send an order to turn D'Erlon for a second time when he was informed that I corps was marching away from Quatre-Bras? What if D'Erlon did use his initiative other than merely obeying the newly arrived order and did pursue only one of the ways insistently and showed up either one of the battle scenes? (Ney's orders reached him when he was just about to arrive Ligny battlefield and deploying for attack.) For sure, the course of Waterloo in a general sense would be different and maybe the next battle would be fought in somewhere else, resulting most probably in favor of the French. If Ney was able to use D'Erlon's corps, he would crush the English at Quatre-Bras and pursue them away from the Prussians. Or, if D'Erlon had made contact with the left wing of Napoleon, most of the Prussian Army, which had already been beaten, would be destroyed and they could only retreat in total disorder towards Namur, far away from the English. I think this action of I corps in 16th played more decisive role than any other action seen in this battle, I think even more than the engagements in Waterloo itself.